Only send renderer resize messages for RWHV sizing caused by

TCV's size allocate message.  This avoids a spurious size-allocate
being sent to the renderers when creating a new tab.  The extra
size-allocate is called by the build-in GtkFixed's size allocate
method.

Another way to think of this is that we only size our RWH directly
and decouple it's sizing from the GTK+ widget's sizing.

BUG=26495

Review URL: http://codereview.chromium.org/363012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31031 0039d316-1c4b-4281-b951-d872f2087c98