Part 1 to rename platform/Widget to platform/FrameViewBase.

Rename Widget to FrameViewBase but leave 'using Widget = FrameViewBase' in Widget.h.
Minimum other changes made to other classes that are using forward declarations.
Following CLs will change clients from importing and using Widget.h to use
FrameViewBase.h

BUG=697351

Review-Url: https://codereview.chromium.org/2722953003
Cr-Commit-Position: refs/heads/master@{#453894}
17 files changed