remove more redundant calls to drawingContext

To call CanvasRenderingContext2D::drawingContext is not cheap. This function 
checks whether the context is lost, whether the imagebuffer surface is valid.

Data from Linux perf tool for Canvas2D benchmarks (FishIETank, SpeedReading, 
Galactic, GM3 vector and bitmap...) shows that rotate/translate/scale/clip 
are frequently called per frame. However, these APIs call drawingContext twice. 
As a result, drawingContext costs 2% ~ 7% of the whole render process in these 
benchmarks. 

This simple CL makes rotate/translate/scale/clip/... pass GraphicsContext 
to realizeSaves() if these callers can. The CL removes redundant calls of 
drawingContext() to improve performance.

BUG=

Review URL: https://codereview.chromium.org/503263002

git-svn-id: svn://svn.chromium.org/blink/trunk@180985 bbb929c8-8fbe-4397-9dbb-9b2b20218538