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This CL add a widget and a root view for Sidebar behind a flag. This is an initial implementation which has only basic feature of sidebar. The detailed behavior will be implemented in succeeding patches.

Design doc: go/cros-sidebar
Mock: go/cros-sidebar-mock (Early State)

Bug: 726250
Change-Id: I3ac1239f8509136eafff16b659e0040e36f702b9
Reviewed-on: https://chromium-review.googlesource.com/748983
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520218}