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

Review URL: https://codereview.chromium.org/630843002
diff --git a/src/gpu/gl/debug/GrGLCreateDebugInterface.cpp b/src/gpu/gl/debug/GrGLCreateDebugInterface.cpp
index 4bd8f99..287a43f 100644
--- a/src/gpu/gl/debug/GrGLCreateDebugInterface.cpp
+++ b/src/gpu/gl/debug/GrGLCreateDebugInterface.cpp
@@ -819,7 +819,7 @@
     //      The solution to this is probably to alter SkDebugGlContext's
     //      "makeCurrent" method to make a call like "makeCurrent(this)" to
     //      the debug GL interface (assuming that the application will create
-    //      multiple SkGLContextHelper's) to let it switch between the active
+    //      multiple SkGLContext's) to let it switch between the active
     //      context. Everything in the GrDebugGL object would then need to be
     //      moved to a GrContextObj and the GrDebugGL object would just switch
     //      between them. Note that this approach would also require that