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This CL makes sub headers in detailed view transparent. As sub headers
in detailed view sticks to the top, we should clip the contents behind
the sub header when it's sticking.

Screenshot (before): http://screen/VdTYhhOnzmB
Screenshot (after): http://screen/42SJVs7U1RF

TEST=manual
BUG=850396

Change-Id: If112681997014c5bf7696dea775d414250e67f39
Reviewed-on: https://chromium-review.googlesource.com/1094853
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565994}