Fixed race in initialization of pinch viewport's container layer size.

The PinchViewport gets its size from WebView::resize. However, in some
initialization paths the WebView gets sized prior to the PinchViewport
being attached to the layer tree so the inner viewport container layer
doesn't get resized until something triggers WebView to resize.

This fix sets the PinchViewport's size even if it hasn't been attached to
the layer tree  yet. It then sets the size of the container layer upon
attachment.

BUG=372018

Review URL: https://codereview.chromium.org/278023003

git-svn-id: svn://svn.chromium.org/blink/trunk@173949 bbb929c8-8fbe-4397-9dbb-9b2b20218538