Clean up CollectLayers code

Now that CollectLayers directly uses FillBounds we can:

skip the explicit intersection with the clipBounds after an adjustAndMap call

skip the storage and use of the clipBounds in SaveLayerInfo

Review URL: https://codereview.chromium.org/719793002
Add more tests.

BUG=skia:

Review URL: https://codereview.chromium.org/717143003
Remove SK_USE_SCALED_FONTMETRICS.

This define was added in
"SK_USE_SCALED_FONTMETRICS for correct scaling"
c17c6582ec0b7bc211d7de9e8d634692d239c123.
Users all now define this flag, so it may now be removed.

BUG=chromium:420901

Review URL: https://codereview.chromium.org/720743003
Sk4x_sse.h

BUG=skia:

Review URL: https://codereview.chromium.org/698873003
Move GrInvariantOutput out of GrProcessor and into its own class.

This will help with the ability to subclass, add to, etc. GrInvariantOutput. Also it was simply
getting a little too big to be a "supporting" subclass
BUG=skia:

Review URL: https://codereview.chromium.org/699943003
Rebaseline for Nexus10 for 6fee786210bd2.