Convert AwViewMsg_SetBackgroundColor to a mojom message.

There are a series of messages that are sent to the LocalMainFrame
of the webview. This maps SetBackgroundColor onto one of those messages.
Other targeted messages will be converted in followup CLs.

BUG=993189

Change-Id: I221d0c9be378c731d0bb3a11e508bda8ba0dc849
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518798


Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824926}