Make AX update layout on all frames

When updating layout before exploring the AX tree, do it on
all frames. Relax the assertion in WebAXObject::boundingBoxRect
so that it's okay if the lifecycle is StyleClean and needsLayout() is false.

Also a bit of cleanup - the name "backing store", inherited from WebKit,
doesn't make sense in this context - changed it to be more clear.

BUG=374275

Review URL: https://codereview.chromium.org/286143008

git-svn-id: svn://svn.chromium.org/blink/trunk@174407 bbb929c8-8fbe-4397-9dbb-9b2b20218538