Increase per-test timeout in WebGL conformance suite to 180s from 120s.

A couple of the tests are close to the 120s limit on the Debug bots.

BUG=409118

Review URL: https://codereview.chromium.org/524033002

Cr-Commit-Position: refs/heads/master@{#292766}

content/test/gpu/gpu_tests/webgl_conformance.py

@@ -92,7 +92,7 @@ class WebglConformancePage(page_module.Page):
   def RunNavigateSteps(self, action_runner):
     action_runner.NavigateToPage(self)
     action_runner.WaitForJavaScriptCondition(
-        'webglTestHarness._finished', timeout_in_seconds=120)
+        'webglTestHarness._finished', timeout_in_seconds=180)
 
 
 class WebglConformance(benchmark_module.Benchmark):