Fix some SVGFonts compilation errors

Fix some SVGFonts compilation errors that are seen when turning off the compile time flag SVG_FONTS.

BUG=

Review URL: https://codereview.chromium.org/329143007

git-svn-id: svn://svn.chromium.org/blink/trunk@175985 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove GraphicsContext::fastMod and ::isDrawingToLayer.

fastMod has been dead code for over one year (since I removed the only call site):
https://chromiumcodereview.appspot.com/14718009

isDrawingToLayer is trivial, and its only call site is inside GraphicsContext.

Review URL: https://codereview.chromium.org/332443003

git-svn-id: svn://svn.chromium.org/blink/trunk@175986 bbb929c8-8fbe-4397-9dbb-9b2b20218538