Fix pixel tests failure when dcheck is on

Fix test failure when binary built with --dcheck_always_on=true.

Bug: 958242
Change-Id: Ic4645e517c0f29169e444fb1b6d3b1b2921720d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063223
Auto-Submit: Sven Zheng <svenzheng@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742780}

ui/base/test/skia_gold_pixel_diff.cc

@@ -194,6 +194,7 @@ void SkiaGoldPixelDiff::Init(const std::string& screenshot_prefix) {
   }
   initialized_ = true;
   prefix_ = screenshot_prefix;
+  base::ScopedAllowBlockingForTesting allow_blocking;
   base::CreateNewTempDirectory(FILE_PATH_LITERAL("SkiaGoldTemp"),
                                &working_dir_);