Expose hide/show methods on tab to call into the correpsonding CVC methods.

Fixes an issue when leaving the native NTP, that the renderer did not have
a strong binding and would die.

BUG=340265

Review URL: https://codereview.chromium.org/175823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252719 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252719