Refactor GrLayerCache for new API

Refactor GrLayerCache for new API

The only substantive change in this CL is skipping atlasing
for any layers that are involved in nesting. Prior versions
have allowed nesting layers to be atlased.

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All times are on Windows with a repeat count of 200.
Format is: <time in ms> (<# of glBindframebuffer calls>)

How to interpret this:

For the boogie page:

 both columns should be the same (since boogie has no
        nested layers)
 without the new API the tiled case doesn't show any real
        benefit from hoisting

For the carsvg page:

 The nesting change does increase the number of FBO
      switches but doesn't kill performance
 Because of the location & size of the layers the
      tile case does show some improvement (even
      without the new API)

boogie
-------

		   w/o nested change   w/ nested change

simple                5.62 (811)           N/A

tiled                 7.72 (811)           N/A

simple w/ hoisting    5.23 (409)        5.77 (409)
(but no caching)

tiled w/ hoisting     7.57 (809)        7.49 (809)
(but no caching)

carsvg
------

		   w/o nested change   w/ nested change

simple 		      60.37 (141990)        N/A

tiled                115.13 (256929)        N/A

simple w/ hoisting    41.57 (64570)      42.82 (72279)
(but no caching)

tiled w/ hoisting     84.24 (154352)     84.71 (165630)
(but no caching)

R=bsalomon@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/476833004