Cleanup GraphicsContext by removing unused functions

This patch removes the following unused functions:
const SkBitmap* bitmap() const;
const SkBitmap& layerBitmap(AccessMode = ReadOnly) const;
void drawImage(Image*, const IntPoint& destPoint, const IntRect& srcRect, CompositeOperator = CompositeSourceOver, RespectImageOrientationEnum = DoNotRespectImageOrientation);
bool hasShadow() const;
static void addCornerArc(SkPath*, const SkRect&, const IntSize&, int);

BUG=

Review URL: https://codereview.chromium.org/182183004

git-svn-id: svn://svn.chromium.org/blink/trunk@168072 bbb929c8-8fbe-4397-9dbb-9b2b20218538