Enable tracing for even more detailed debug log capturing

This enables very detailed logging of network activity, page execution
and script execution for 7 seconds after startup.

This is part of debugging frequent CI build failures in the client repo
on Jenkins. From the Karma logs I can see that all of the test files are
requested from the Karma server, but none of the log messages associated
with test execution are seen. This tracing should make it possible to
find out which JS scripts even execute.

src/karma.config.js

@@ -28,6 +28,16 @@ if (process.env.TRAVIS || process.env.RUNNING_IN_DOCKER) {
     __dirname + '/../',
     'chrome_debug.log'
   );
+
+  // Enable even more debug logging from Chrome to help track down build
+  // failures in Jenkins. This generates a large (~40MB+) JSON file detailing
+  // events that happen during browser startup.
+  //
+  // See https://www.chromium.org/developers/how-tos/trace-event-profiling-tool/recording-tracing-runs#TOC-Capturing-chrome-desktop-startup
+  const traceFile = path.resolve(__dirname + '/../', 'chrome_trace.json');
+  chromeFlags.push('--trace-startup');
+  chromeFlags.push('--trace-startup-duration=7');
+  chromeFlags.push(`--trace-startup-file=${traceFile}`);
 }
 
 if (process.env.RUNNING_IN_DOCKER) {