Temporarily skip Pixel_OffscreenCanvasWebGLDefault on Fuchsia.

Flaking frequently on SkiaRenderer-Vulkan, causing CQ retries.

Bug: 1157136
Change-Id: I64ec1665940b993f135797449d5bfabb7de66874
Tbr: juanmihd@chromium.org
Tbr: fserb@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2592148Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836949}

content/test/gpu/gpu_tests/test_expectations/pixel_expectations.txt

@@ -109,6 +109,7 @@ crbug.com/1115673 [ fuchsia ] Pixel_WebGLCopyImage [ Skip ]
 crbug.com/1115673 [ fuchsia ] Pixel_CanvasLowLatencyWebGLDrawImage [ Skip ]
 crbug.com/1115673 [ fuchsia ] Pixel_CanvasLowLatencyWebGLRoundedCorners [ Skip ]
 crbug.com/1115673 [ fuchsia ] Pixel_CanvasLowLatencyWebGLOccluded [ Skip ]
+crbug.com/1157136 [ fuchsia skia-renderer-vulkan ] Pixel_OffscreenCanvasWebGLDefault [ Skip ]
 
 # Tests running with SwiftShader are skipped on platforms where SwiftShader
 # isn't supported.