Renamed and made public SkGradientShaderBases's 'commonAsAGradient' to 'getGradientTa...

Renamed and made public SkGradientShaderBases's 'commonAsAGradient' to 'getGradientTableBitmap', and use that instead of asABitmap in gradient custom stage setup.

Also tidied up Gr gradient implementation constructors, to take the appropriate SkGradientShaderBase subclass, and where necessary (namely 2pt radial/conical) made them obtain extra parameters from that object, rather than passing them in in addition to it.
Review URL: https://codereview.appspot.com/6449057

git-svn-id: http://skia.googlecode.com/svn/trunk@4808 2bbb7eff-a529-9590-31e7-b0007b416f81