android: Introduce a ui::PlatformWindow implementation for android.
This change introduces a ui::PlatformWindow implementation for android, and uses
that from view_manager. The implementation of PlatformWindowAndroid is copied over
from PlatformViewportAndroid. The various PlatformViewport implementations are
merged into a single PlatformViewportCommon implementation, that uses the common
ui::PlaformWindow abstraction for managing the windows.
BUG=487881
Committed: https://crrev.com/
870d6cf7d409b5e7dae82c98bfaeaa5137a652d9
Cr-Commit-Position: refs/heads/master@{#334262}
Review URL: https://codereview.chromium.org/
1180573004
Cr-Commit-Position: refs/heads/master@{#334359}