Tab manager view should hide on press of back key

Currently when tab manager is shown and user presses back key
then instead of hiding tab manager page is navigated to previous
entry. If there is nothing to navigate then application is exited.
Added code to hide tab manager view if back key is pressed and tab
manager view is on.

BUG=

Review URL: https://codereview.chromium.org/689583002

Cr-Commit-Position: refs/heads/master@{#302222}
3 files changed