On Nexus 5X, Nexus 7, and other devices that don't use one of the
standard densities (mdpi, hdpi, etc), the top of the infobar faded from
white to gray (over a few pixel rows) instead of having an immediate
jump from white to gray. This happened because the 9-patch infobar
background was resized, causing the sharp edge to become blurry.

As a fix, this CL uses a layer list as the infobar background with two
layers: a bitmap for the infobar shadow, and solid white the rest of the
background. Using a layer list ensures that the infobar shadow's bounds
will be pixel-aligned.

BUG=396223

Review URL: https://codereview.chromium.org/1532793002

Cr-Commit-Position: refs/heads/master@{#365725}