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This is to speed up compile of headless_browser_impl.
On linux, compile time of  headless_browser_impl.cc reduced from 22 seconds to 8 seconds.


Change-Id: I0645dfc245894e83762711f419d9634e57077ccb
Reviewed-on: https://chromium-review.googlesource.com/1245000Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594527}