Conditionalize new code which fetches stack traces upon failure.

BUG=609252
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel
NOTRY=true

Review-Url: https://codereview.chromium.org/2085213003
Cr-Commit-Position: refs/heads/master@{#401509}

content/test/gpu/gpu_tests/webgl_conformance.py

@@ -84,8 +84,13 @@ def __init__(self):
 
   def ValidateAndMeasurePage(self, page, tab, results):
     if not _DidWebGLTestSucceed(tab):
-      is_valid_dump, trace_output = tab.browser.GetStackTrace()
       messages = _WebGLTestMessages(tab)
+      is_valid_dump = False
+      # Problems have been seen attempting to get stack traces on
+      # Android via this API; see crbug.com/609252. Skip this logic
+      # there for the time being.
+      if tab.browser.platform.GetOSName() != 'android':
+        is_valid_dump, trace_output = tab.browser.GetStackTrace()
       if is_valid_dump:
         messages += '\nStack Trace:\n' + trace_output
       raise page_test.Failure(messages)