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

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:

Review URL: https://codereview.chromium.org/777003002