[Instant Start] Update Feed Placeholder layout.

Update Feed placeholder layout for a lighter border and
use the screen width to assign the width of placeholders.

Bug: 1076139
Change-Id: I6080cbf2362c701578004c091439be05c565420c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311424


Commit-Queue: Hao Dong <spdonghao@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791471}