Use ContainsValue() instead of std::find() in content/

BUG=561800

Change-Id: Ia074881a22e83873ff26249ea056ad9c6365fe14
Reviewed-on: https://chromium-review.googlesource.com/c/1248141
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Cr-Commit-Position: refs/heads/master@{#599838}
23 files changed