Ignore SkGlyphCache gamma for nvpr text

BUG=skia:

Review URL: https://codereview.chromium.org/740343006
Use variable length key (rather than accumulated matrix) as save layer hoisting key

Adding the rendering canvas' CTM to the layer hoisting key (i.e., Add support for hoisting layers in pictures drawn with a matrix - https://codereview.chromium.org/748853002/) has increased the cache miss rate due to accumulated floating point error. This CL fixes part of the issue by using the chain of operation indices leading to each saveLayer as the key. The canvas' CTM must still form part of the key but should be less subject to accumulated error.

BUG=skia:2315

Review URL: https://codereview.chromium.org/753253002
Remove SK_SUPPORT_LEGACY_DEEPFLATTENING.

This was needed for pictures before v33, and we're now requiring v35+.

Will follow up with the same for skia/ext/pixel_ref_utils_unittest.cc

BUG=skia:

Review URL: https://codereview.chromium.org/769953002