Chrome Version: 61
OS: Mac

What steps will reproduce the problem?
(1) Patch in https://chromium-review.googlesource.com/c/578915/ which adds a thread checker to gfx::Image.
(2) From a Debug build, run unit_tests --gtest_filter='WebAppShortcutCreatorTest.*'

What is the expected result?
No fail.

What happens instead?

 RUN      ] WebAppShortcutCreatorTest.RunShortcut
[52123:25091:0719/234505.722607:36901941352984:FATAL:image.cc(365)] Check failed: AccessIsThreadSafe().
0   unit_tests                          0x0000000106aa145c base::debug::StackTrace::StackTrace(unsigned long) + 28
1   unit_tests                          0x0000000106ac4a40 logging::LogMessage::~LogMessage() + 224
2   unit_tests                          0x000000010754b29d gfx::Image::AsNSImage() const + 109
3   unit_tests                          0x0000000108ffe5e5 (anonymous namespace)::ImageRepForGFXImage(gfx::Image const&) + 21
4   unit_tests                          0x0000000108ffd946 (anonymous namespace)::SetWorkspaceIconOnFILEThread(base::FilePath const&, std::unique_ptr<ResourceIDToImage>) + 230

The sequence that leads to this crash:
1. FILE thread: UpdateAppShortcutsSubdirLocalizedName PostTask to UI thread: GetImageResourcesOnUIThread
2. UI thread: GetImageResourcesOnUIThread calls ui::ResourceBundle::GetNativeImageNamed(), retrieving gfx::Image instances shared with static references on the UI thread.
3. FILE thread: SetWorkspaceIconOnFILEThread calls gfx::Image::AsNSImage on these images.

AsNSImage is not thread-safe because other code running simultaneously on the UI thread may be updating the image cache.

Possible solutions:
A. Call AsNSImage in GetImageResourcesOnUIThread, and pass a std::map<int, NSImage*> across to SetWorkspaceIconOnFILEThread instead.
B. Do what tapted@ suggested in  https://crbug.com/742145#c12  and make the gfx::Image update its internal cache atomically.

A is my preferred option as it avoids all thread safety issues in gfx::Image; the latter only works because SetWorkspaceIconOnFILEThread never copies a gfx::Image object.