DevTools: [CanvasProfiler] Protocol API to get the current canvas context state.

BUG=253854
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/19227002

git-svn-id: svn://svn.chromium.org/blink/trunk@154567 bbb929c8-8fbe-4397-9dbb-9b2b20218538