Clip in grdrawtarget

NOTREECHECKS=true
BUG=skia:

Review URL: https://codereview.chromium.org/685883003
Get gpudft support working in dm, gm, nanobench and bench_pictures

Adds a new config to test distance field text.
Clean up some flags and #defines to read "distance field text",
not "distance field fonts" to be consistent with Chromium

NOTREECHECKS=true

Review URL: https://codereview.chromium.org/699453005
Revert of Get gpudft support working in dm, gm, nanobench and bench_pictures (patchset #2 id:20001 of https://codereview.chromium.org/699453005/)

Reason for revert:
Not compiling in ANGLE build

Original issue's description:
> Get gpudft support working in dm, gm, nanobench and bench_pictures
>
> Adds a new config to test distance field text.
> Clean up some flags and #defines to read "distance field text",
> not "distance field fonts" to be consistent with Chromium
>
> NOTREECHECKS=true
>
> Committed: https://skia.googlesource.com/skia/+/06ba179838ba4fe187cf290750aeeb4a02a2960b

TBR=bsalomon@google.com,mtklein@google.com,reed@google.com
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/707723005