[glic] Use `SOLID_COLOR_LAYER` for background of GlicView on ChromeOS

GlicView (i.e the WebView), paints a paints onto the widget's texture
layer. This background is shows before the WebContents are ready to
start rendering content.

It is more efficient to use `SOLID_COLOR_LAYER` here as we do not to
recreate the texture every time window resizes. Additionally, it is more
efficient to use shader based rounded corners than painting a rounded
background.

Bug: b:458506119, b:459890634
Change-Id: I0e509d5739ab88557bd8588d40f0e1f7147a63b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7148097
Commit-Queue: Zoraiz Naeem <zoraiznaeem@chromium.org>
Reviewed-by: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1543981}