Avoid testing antialiased boundary in ScreenshotSyncIntegrationTest.

Skia currently has a driver workaround that performans antialiased
rectangle edge equation evalution in the fragment shader rather than
vertex shader on Adreno 3xx GPUs. This was done because the interpolation
of values computed in the vertex shader lacks enough precision to pass
these tests. However, doing so has a negative performance impact. Skia
tests run with the workaround removed show that in practice the visual
difference is insignificant. Moreover, cc::GLRenderer also incurs these
same errors as it uses the same antialiasing approach without the
workaround and has shipped on Adreno 3xx devices for years with no known
complaints about antialiasing quality/accuracy. We're better off
reclaiming the performance and accepting a small hit to antialiasing
accuracy.

Bug: 847984
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: If1e62e0780a752179a75ddd57ccc7b24128c68ac
Reviewed-on: https://chromium-review.googlesource.com/1129041Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#573695}

content/test/gpu/gpu_tests/screenshot_sync_integration_test.py

@@ -119,9 +119,12 @@ class ScreenshotSyncIntegrationTest(gpu_integration_test.GpuIntegrationTest):
         "window.draw({{ red }}, {{ green }}, {{ blue }});",
         red=canvasRGB.r, green=canvasRGB.g, blue=canvasRGB.b)
     screenshot = tab.Screenshot(10)
-    start_x = 10
-    start_y = 0
-    outer_size = 256
+    # Avoid checking along antialiased boundary due to limited Ardeno 3xx
+    # interpolation precision (crbug.com/847984).
+    inset = 1
+    start_x = inset
+    start_y = inset
+    outer_size = 256 - inset
     skip = 10
     for y in range(start_y, outer_size, skip):
       for x in range(start_x, outer_size, skip):