BindingManager: fix an edge-case of the background period binding.

Since http://crrev.com/237333, BindingManager is responsible for keeping an
additional, strong binding on the foreground renderer while the embedder
application remains in background.

To implement that, we reused the existing |mLastOomBoundPid| field that keeps
track of the renderer that was most recently bound with an oom binding. When
the app goes to background, we put the additional binding on whatever is
indicated by |mLastOomBoundPid|.

But, |mLastOomBoundPid| is set not only when a strong binding is added for a
connection, but also when a new connection (holding the initial binding) is
created. That means that it will point to a background rendererer right after a
tab is opened and loaded in background.

Therefore it is currently possible to have BindingManager attach the background
period binding on a wrong renderer.

This patch fixes that by not setting the |mLastOomBoundPid| when a new
connection is started. We can do that, because:
- on the high-end we use the field only for the background period binding
- on the low-end a new renderer is always started in foreground, so the values
  stored in the field will not change

Testing for the BindingManager is also modified to cover this case.

BUG=317963

Review URL: https://codereview.chromium.org/108203004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239848 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239848