Allow extensions to hide bookmark star

Implements the Chrome extension API proposal for hiding the bookmark
star for the purpose of allowing extensions to provide alternate UI for
creating bookmarks. For example: using a page action to display bookmark
state and create/edit a bookmark in place of the built-in star button.

This CL implements the star hiding behavior for Views. A Cocoa implementation
will be done in a follow-on CL.

To enable this behavior, the following must hold:
 - the flag --enable-override-bookmarks-ui=1 is passed on the command line
 - an enabled extension has a manifest with the section
    "chrome_settings_overrides": {
      "bookmarks_ui": {
	"hide_bookmark_button": true
      }
    }

The API proposal document is at:
https://docs.google.com/a/chromium.org/document/d/17YldHwrDBZ2L5lXWo7eDff_oZIrwNF-8Uk2fPLHU4po/edit


BUG=311282

Review URL: https://codereview.chromium.org/93423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240286 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240286