Rename platform/Widget to platform/FrameViewBase in core.

BUG=697351
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2723993002
Cr-Commit-Position: refs/heads/master@{#453912}
35 files changed