Fix Views Omnibox paste handling and menu availability.

Check the Textfield's controller for command state if it's the handler.
(allows the Omnibox to disable the context menu's paste command)
Check OmniboxView::GetClipboardText validity for paste enabling.
Handle Shift+Insert for paste (otherwise the textfield will do it).

BUG=296881
TEST="Copy Image" from web content, then right click in the omnibox, paste is disabled, also Shift+Insert doesn't paste an unexpected HTML fragment.
R=pkasting@chromium.org

Review URL: https://codereview.chromium.org/38603003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231430 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231430