Make TabBase non abstract

TabBase will build it's own native component if nobody overrides initializeNative().

BUG=277135

Review URL: https://codereview.chromium.org/108803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243160 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243160