Update backbutotn state

Disable/enable the button based on navigation state.

The appearance will be adjusted in a separate CL.

BUG=749713
TEST=Covered by unit test.

Change-Id: Id577c3971440bebfebb2490c7398870cd4f68c93
Reviewed-on: https://chromium-review.googlesource.com/843821
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528162}
4 files changed