TranslateManager is no longer a singleton

There is now a TranslateManager per tab.
TranslateManager is owned by TranslateTabHelper.

This changes simplifies all the tracking of WebContents objects,
since there is now one and only one WebContents per TranslateManager.
This allows to get rid of all the |web_contents| parameters as well as the
PendingRequest type.

Users of the TranslateManager now get a manager from a WebContents
object, through the associated TranslateTabHelper.

BUG=332736
TBR=brettw

Review URL: https://codereview.chromium.org/166963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252098 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252098