Tickle the GL context during context loss test to ensure loss event.

Seems that we may not get the webglcontextlost event unless a GL command
happens to fail after the context loss has occurred. Use a 250ms timer
to tickle the context until the webglcontextlost event occurs.

BUG=929012,929429

Change-Id: I191b2338f633612fc3b8206bf27f9a14c968bd23
Reviewed-on: https://chromium-review.googlesource.com/c/1459780
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630134}

content/test/data/gpu/pixel_video_context_loss.html

@@ -39,8 +39,14 @@ function CrashGpuProcess() {
   var canvas = document.createElement('canvas');
   var gl = canvas.getContext('webgl');
 
+  // webglcontextlost may not be invoked unless we actually try to execute a GL
+  // command, so use a repeating function to tickle the context.
+  var tickle = setInterval(function() { gl.getParameter(gl.VERSION); }, 250);
+
   // Now wait for the context loss before starting video playback.
   canvas.addEventListener('webglcontextlost', function(e) {
+    clearInterval(tickle);
+
     // Now wait for the video to play until the end, by which point a frame
     // should be visible on screen.
     video.addEventListener('ended', function(e) {