Get rid of the call to updateDrawsContent in CompositedLayerMapping::updateCompositedBounds.

This should no longer be necessary. It was added in crrev.com/68833004 to address an issue in
which descendant layers have inconsistent state about painting into the same backing.
However, subsequent refactoring simplified the implementation of
RenderLayer::boundingBoxForCompositing significantly (e.g. crrev.com/226043007), and I belive
also removed the need for this code.

BUG=372837

Review URL: https://codereview.chromium.org/296483010

git-svn-id: svn://svn.chromium.org/blink/trunk@174504 bbb929c8-8fbe-4397-9dbb-9b2b20218538