Make the Sk GL context class an abstract base class

Make the Sk GL context class, SkGLNativeContext, an abstract base class. Before,
it depended on ifdefs to implement the platform dependent polymorphism.  Move
the logic to subclasses of the various platform implementations.

This a step to enable Skia embedders to compile dm and bench_pictures. The
concrete goal is to support running these test apps with Chromium command buffer.

With this change, Chromium can implement its own version of SkGLNativeContext
that uses command buffer, and host the implementation in its own repository.

Implements the above by renaming the SkGLContextHelper to SkGLContext and
removing the unneeded SkGLNativeContext. Also removes
SkGLNativeContext::AutoRestoreContext functionality, it appeared to be unused:
no use in Skia code, and no tests.

BUG=skia:2992

Review URL: https://codereview.chromium.org/630843002
Make GM default to reseting gpu contexts on android.

TBR=mtklein@google.com

Review URL: https://codereview.chromium.org/635293003
speculative fix for ios

TBR=

Review URL: https://codereview.chromium.org/637293003
Revert of speculative fix for ios (patchset #1 id:1 of https://codereview.chromium.org/637293003/)

Reason for revert:
Need to revert a90ed4e83897b45d6331ee4c54e1edd4054de9a8

Original issue's description:
> speculative fix for ios
>
> TBR=
>
> Committed: https://skia.googlesource.com/skia/+/96e89d5b45e8e293139ecd95302b5b4f07f8e9dc

TBR=
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/636153002
Revert of Make the Sk GL context class an abstract base class (patchset #4 id:60001 of https://codereview.chromium.org/630843002/)

Reason for revert:
nanobech failing on Android

Original issue's description:
> Make the Sk GL context class an abstract base class
>
> Make the Sk GL context class, SkGLNativeContext, an abstract base class. Before,
> it depended on ifdefs to implement the platform dependent polymorphism.  Move
> the logic to subclasses of the various platform implementations.
>
> This a step to enable Skia embedders to compile dm and bench_pictures. The
> concrete goal is to support running these test apps with Chromium command buffer.
>
> With this change, Chromium can implement its own version of SkGLNativeContext
> that uses command buffer, and host the implementation in its own repository.
>
> Implements the above by renaming the SkGLContextHelper to SkGLContext and
> removing the unneeded SkGLNativeContext. Also removes
> SkGLNativeContext::AutoRestoreContext functionality, it appeared to be unused:
> no use in Skia code, and no tests.
>
> BUG=skia:2992
>
> Committed: https://skia.googlesource.com/skia/+/a90ed4e83897b45d6331ee4c54e1edd4054de9a8

TBR=kkinnunen@nvidia.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:2992

Review URL: https://codereview.chromium.org/639793002
Expose layer hoisting API in GrContext

This CL exposes the layer hoisting API in GrContext for use in SkMultiPictureDraw::draw. It basically mirrors what SkGpuDevice::EXPERIMENTAL_drawPicture has been doing.

Review URL: https://codereview.chromium.org/533673002
Add SkCachedData and use it for SkMipMap

This reverts commit 37c5a815d8ea33247968212ef4cc83394ceee1bc.

TBR=mtklein

Review URL: https://codereview.chromium.org/635333002