Version: 
OS: Win

What is the expected output? What do you see instead?
This compile error happened flakily and closed the tree, without any relation to the CL that triggered the build. ui/gfx/vector_icons.h is generated by https://code.google.com/p/chromium/codesearch#chromium/src/ui/gfx/vector_icons/aggregate_vector_icons.py&q=vector_icons.h.template&sq=package:chromium&type=cs, and I would guess https://code.google.com/p/chromium/codesearch#chromium/src/components/toolbar/test_toolbar_model.cc&sq=package:chromium&type=cs&q=test_toolbar_model.cc&l=1 does not properly depend on the generated header.

https://build.chromium.org/p/chromium/builders/Win/builds/41515/steps/compile/logs/stdio:

[10395/32767] CXX obj\ui\events\test\events_test_support.platform_event_source_test_api.obj
FAILED: ninja -t msvc -e environment.x86 -- C:\b\build\goma/gomacc "C:\b\depot_tools\win_toolchain\vs_files\a3796183a9fc4d22a687c5212b9c76dbd136d70d\VC\bin\amd64_x86\cl.exe" /nologo /showIncludes /FC @obj\components\toolbar\toolbar_test_support.test_toolbar_model.obj.rsp /c ..\..\components\toolbar\test_toolbar_model.cc /Foobj\components\toolbar\toolbar_test_support.test_toolbar_model.obj /Fdobj\components\toolbar_test_support.cc.pdb 
c:\b\build\slave\win\build\src\ui\gfx\vector_icons_public.h(11): fatal error C1083: Cannot open include file: 'ui/gfx/vector_icons.h': No such file or directory
ninja: build stopped: subcommand failed.
[10395/32767] CXX obj\ui\events\test\events_test_support.platform_event_source_test_api.obj
FAILED: ninja -t msvc -e environment.x86 -- C:\b\build\goma/gomacc "C:\b\depot_tools\win_toolchain\vs_files\a3796183a9fc4d22a687c5212b9c76dbd136d70d\VC\bin\amd64_x86\cl.exe" /nologo /showIncludes /FC @obj\components\toolbar\toolbar_test_support.test_toolbar_model.obj.rsp /c ..\..\components\toolbar\test_toolbar_model.cc /Foobj\components\toolbar\toolbar_test_support.test_toolbar_model.obj /Fdobj\components\toolbar_test_support.cc.pdb 
c:\b\build\slave\win\build\src\ui\gfx\vector_icons_public.h(11): fatal error C1083: Cannot open include file: 'ui/gfx/vector_icons.h': No such file or directory
ninja: build stopped: subcommand failed.

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