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Clear the DWM frame area on WM_ERASEBKGND so that Chrome's client area
blends with DWM frame for app windows.  Refactor the code to update and
clear DWM frame and move it to hwnd message handler so that browser and
app windows can share the same code.

This mimics existing logic for updating the DWM frame and adds the clear
DWM frame behavior to app windows, but one notable change is that it
will clear on every WM_ERASEBKGND message, and not just the first one.
This shouldn't have a performance impact and seems more correct anyway.

Bug: 904322
Change-Id: I70c3ae97a94114dd63110736a35cef4dd887b1aa
Reviewed-on: https://chromium-review.googlesource.com/c/1382928Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618684}