Add stubs for setRenderingContext and setShouldFlattenTransform

Landing this in advance of https://codereview.chromium.org/98373011/
will allow the new API functions being added to WebLayer to be pure
virtual. The real implementation of these functions is coming in
https://codereview.chromium.org/147833003/

R=jamesr@chromium.org
BUG=322216,338980

Review URL: https://codereview.chromium.org/145973018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248719 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248719