Move all data members from WebContents to TabContents in preparation for

      merging the two. This is extremely straightforward except for the
GearsCreateShortcutCallbackFunctor which I had to move to the header (with a
TODO and its associated function OnGearsCreateShortcutDone) and some of the
constructors for the members which took a WebContents that I had to cast.

I tried to reorganize the data to have some kind of groupings.

I made the member initializers all explicit for TabContents because the
extreme number of them makes them difficult to keep track of.
Review URL: http://codereview.chromium.org/88021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@14230 0039d316-1c4b-4281-b951-d872f2087c98