Refactor GrGpu path rendering functions to GrPathRendering

GrGpu, GrGLGpu, GrPathRendering, GrGLPathRendering all duplicated
each others' path rendering related member functions. Make
GrPathRending the logical extension of GrGpu by removing the methods
from GrGpu. Similarly to the GL variants.

Changes includes and forward declarations due to removing
GrDrawTarget.h include from GrGpu.h. This was necessary due to
GrDrawTarget.h including GrPathRendering.h, and now GrPathRendering.h
includes GrGpu.h. (Also GrGpu.h does not need GrDrawTarget.h anymore).

Very slight runtime improvement on x86_64:
             desk_fontwipe.skp_1	 836us ->  841us	1.01x
            desk_silkfinance.skp_1	2.01ms -> 2.01ms	1x
             desk_forecastio.skp_1	7.03ms -> 7.05ms	1x
                desk_weather.skp_1	3.74ms -> 3.74ms	1x
                desk_twitter.skp_1	8.02ms -> 8.01ms	1x
                 desk_mapsvg.skp_1	5.24ms -> 5.23ms	1x
            desk_pokemonwiki.skp_1	7.06ms -> 7.03ms	1x
                 desk_sfgate.skp_1	3.19ms -> 3.17ms	1x
           tabl_cuteoverload.skp_1	2.66ms -> 2.64ms	0.99x
...
           tabl_transformice.skp_1	3.06ms -> 2.98ms	0.98x
         tabl_googlecalendar.skp_1	11.3ms ->   11ms	0.97x
            tabl_gamedeksiam.skp_1	12.8ms -> 12.4ms	0.97x
               desk_samoasvg.skp_1	10.6ms -> 10.3ms	0.97x
           tabl_worldjournal.skp_1	4.44ms ->  4.3ms	0.97x

Review URL: https://codereview.chromium.org/1157683006
20 files changed