Increase GPU Maps test threshold

Doubles the tolerance for the GPU Maps pixel test from 3 to 6. This
should hopefully get the test passing more consistently on the NVIDIA
Shield devices.

TBR=ynovikov@chromium.org

Bug: 1043129
Change-Id: I45f87c37814825788690c830f3c9858257bd91b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2165715Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762547}

content/test/gpu/gpu_tests/maps_integration_test.py

@@ -20,7 +20,7 @@ _MAPS_PERF_TEST_PATH = os.path.join(
 _DATA_PATH = os.path.join(path_util.GetChromiumSrcDir(),
                          'content', 'test', 'gpu', 'gpu_tests')
 
-_TOLERANCE = 3
+_TOLERANCE = 6
 
 class MapsIntegrationTest(
     skia_gold_integration_test_base.SkiaGoldIntegrationTestBase):