Revert of Revert of remove unused device::eraseColor (patchset #1 id:1 of https:...

Revert of Revert of remove unused device::eraseColor (patchset #1 id:1 of https://codereview.chromium.org/777003002/)

Reason for revert:
fix landed in chrome to allow this (I hope)

Original issue's description:
> Revert of remove unused device::eraseColor (patchset #1 id:1 of https://codereview.chromium.org/765643004/)
>
> Reason for revert:
> ../../skia/ext/bitmap_platform_device_cairo.cc:131:13: error: no member named 'eraseColor' in 'skia::BitmapPlatformDevice'
>
>
> Original issue's description:
> > remove unused device::eraseColor
> >
> > BUG=skia:
> > TBR=
> >
> > Committed: https://skia.googlesource.com/skia/+/b1ab276c3719d6690e58f347c25126d2b8929c5d
>
> TBR=reed@google.com
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/8b0197ef2e32be9d62e79d58f8b50811975c79c6

TBR=halcanary@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/779543004