Have LiveNodeList::rootNode() return a reference

Have LiveNodeList::rootNode() return a reference as it can never
return null. This makes the calling code look safer.

BUG=

Review URL: https://codereview.chromium.org/29873003

git-svn-id: svn://svn.chromium.org/blink/trunk@160143 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move CrossThreadCopier.cpp/.h to platform/

BUG=297477

Review URL: https://codereview.chromium.org/31483002

git-svn-id: svn://svn.chromium.org/blink/trunk@160144 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move TransformSkMatrix44Conversions.cpp/.h to platform/

BUG=297477

Review URL: https://codereview.chromium.org/27332004

git-svn-id: svn://svn.chromium.org/blink/trunk@160145 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Do not show the screencast button in the frontend if it is not supported

BUG=304707

Review URL: https://codereview.chromium.org/28473003

git-svn-id: svn://svn.chromium.org/blink/trunk@160146 bbb929c8-8fbe-4397-9dbb-9b2b20218538