We were messing up the client rect on resize, which caused the
ReadPixels checks to mess up around the window edges. Disabling
the window styles on the test windows masked this bug. Fix this
by using a style-less child window inside the parent window.
This gives us access to window styles for the samples project,
along with the ability to use tiny 1x1 windows for testing.

BUG=angle:730

Change-Id: Ic6dd931df7b4e32fbbcacbb004d3bbc49917f658
Reviewed-on: https://chromium-review.googlesource.com/217024Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>