Changed how we log uncaught exceptions by the WebGL tests

This allows us to output the exception message even if no test has explicitly failed.

BUG=407976
R=kbr@chromium.org
TBR=kbr@chromium.org

Review URL: https://codereview.chromium.org/515513003

Cr-Commit-Position: refs/heads/master@{#292250}

content/test/gpu/gpu_tests/webgl_conformance.py

@@ -53,10 +53,7 @@ conformance_harness_script = r"""
   window.parent.webglTestHarness = testHarness;
   window.console.log = testHarness.log;
   window.onerror = function(message, url, line) {
-    testHarness._failures++;
-    if (message) {
-      testHarness.log(message);
-    }
+    testHarness.reportResults(null, false, message);
     testHarness.notifyFinished(null);
   };
 """