Adding off by one comparators in cc_unittests

SwiftShader's Vulkan library will soon switch from using LLVM as its
backend to Subzero. When using Subzero small off by one errors may
happen, so a few off by one comparators were added in both cases where
this was happening.

Bug: 1060139
Change-Id: Ib3b793f860cb7d76ef8fd1846757e7ae1e483f28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097178Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Alexis Hetu <sugoi@google.com>
Cr-Commit-Position: refs/heads/master@{#748907}

cc/trees/layer_tree_host_pixeltest_filters.cc

@@ -452,6 +452,9 @@ TEST_P(LayerTreeHostBlurFiltersPixelTestGPULayerList,
       average_error_allowed_in_bad_pixels,
       large_error_allowed,
       small_error_allowed));
+#else
+  if (use_vulkan())
+    pixel_comparator_ = std::make_unique<FuzzyPixelOffByOneComparator>(true);
 #endif
 
   RunPixelTestWithLayerList(
@@ -941,6 +944,9 @@ TEST_P(LayerTreeHostFiltersPixelTest, RotatedDropShadowFilter) {
       percentage_pixels_large_error, percentage_pixels_small_error,
       average_error_allowed_in_bad_pixels, large_error_allowed,
       small_error_allowed));
+#else
+  if (use_vulkan())
+    pixel_comparator_ = std::make_unique<FuzzyPixelOffByOneComparator>(true);
 #endif
 
   RunPixelTest(