Tidy up the comments and variable naming relating to Chrome extension install

Having a variable called 'isChrome' and then explaining why
we are trying to work around Edge's spoofing that aims
to deal with exactly this kind of check is silly.

Instead make it more explicit that we are checking for
support for Chrome _extensions_. If Microsoft Edge supports
all the APIs we need in future for that, great, we'll let
them use it.

h/static/scripts/installer-controller.js

@@ -15,13 +15,18 @@ function showSupportedInstallers(rootElement) {
     }
   }
 
-  var canInstallChromeExt = uaDetect.isChrome && !uaDetect.isMobile;
-  var canInstallBookmarklet = !uaDetect.isChrome && !uaDetect.isMobile &&
-                              !uaDetect.isMicrosoftEdge;
+  // check for Chrome extension support. We also check here for !mobile
+  // so that the page is correct when testing in Chrome dev tools with
+  // mobile device simulation enabled
+  var offerChromeInstall = uaDetect.chromeExtensionsSupported &&
+                           !uaDetect.isMobile;
+  var offerBookmarkletInstall = !offerChromeInstall &&
+                                !uaDetect.isMobile &&
+                                !uaDetect.isMicrosoftEdge;
 
-  showIf('.js-install-chrome', canInstallChromeExt);
-  showIf('.js-install-bookmarklet', canInstallBookmarklet);
-  showIf('.js-install-any', canInstallChromeExt || canInstallBookmarklet);
+  showIf('.js-install-chrome', offerChromeInstall);
+  showIf('.js-install-bookmarklet', offerBookmarkletInstall);
+  showIf('.js-install-any', offerChromeInstall || offerBookmarkletInstall);
 }
 
 module.exports = {

h/static/scripts/test/installer-controller-test.js

@@ -35,19 +35,21 @@ describe('installer page', function () {
   }
 
   it('shows the chrome extension to desktop Chrome users', function () {
-    var controller = createController({isChrome: true});
+    var controller = createController({chromeExtensionsSupported: true});
     assert.isFalse(isHidden(extensionBtn));
     assert.isTrue(isHidden(bookmarkletBtn));
   });
 
   it('shows the bookmarklet on desktop browsers', function () {
-    var controller = createController({isChrome: true, isMobile: false});
-    assert.isFalse(isHidden(extensionBtn));
-    assert.isTrue(isHidden(bookmarkletBtn));
+    var controller = createController({chromeExtensionsSupported: false,
+                                       isMobile: false});
+    assert.isTrue(isHidden(extensionBtn));
+    assert.isFalse(isHidden(bookmarkletBtn));
   });
 
   it('shows only the Via link to mobile browsers', function () {
-    var controller = createController({isChrome: true, isMobile: true});
+    var controller = createController({chromeExtensionsSupported: false,
+                                       isMobile: true});
     assert.isTrue(isHidden(extensionBtn));
     assert.isTrue(isHidden(bookmarkletBtn));
   });

h/static/scripts/ua-detect.js

@@ -2,8 +2,10 @@ module.exports = {
   isMobile: navigator.userAgent.match(/\b(Mobile)\b/),
   isMicrosoftEdge: navigator.userAgent.match(/\bEdge\b/),
 
-  // we test for the existence of window.chrome here because Microsoft Edge
-  // (and possibly others) serve
-  isChrome: typeof window.chrome !== 'undefined' &&
-            typeof window.chrome.webstore !== 'undefined',
+  // we test for the existence of window.chrome.webstore
+  //  here because Microsoft Edge (and possibly others) include
+  // "Chrome" in their UA strings for website compatibility reasons
+  // and Edge also provides an empty "window.chrome" object.
+  chromeExtensionsSupported: typeof window.chrome !== 'undefined' &&
+                             typeof window.chrome.webstore !== 'undefined',
 };