remove redundant call to drawingContext in CanvasRenderingContext2D::drawImage. 

drawingContext is not light-weight at all. It checks whether the context is lost, 
and whether the ImageBufferSurface is valid. If lots of drawImage is called 
per frame, say 1000 fishes for FishIETank benchmark, drawingContext will 
be a hotspot. profiling content_shell on Nexus 7 by linux perf tool, data shows 
that the CPU cycles cost by drawingContext will be 5+% of the whole render process. 
However, drawImage calls drawingContext twice: one is in drawImage, another is 
in drawImageInternal. This simple CL removes one redundant call to improve performance.

Review URL: https://codereview.chromium.org/490423002

git-svn-id: svn://svn.chromium.org/blink/trunk@180838 bbb929c8-8fbe-4397-9dbb-9b2b20218538