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
diff --git a/src/gpu/gl/mesa/SkMesaGLContext.cpp b/src/gpu/gl/mesa/SkMesaGLContext.cpp
index 8c339c7..31402c5 100644
--- a/src/gpu/gl/mesa/SkMesaGLContext.cpp
+++ b/src/gpu/gl/mesa/SkMesaGLContext.cpp
@@ -11,6 +11,24 @@
 #include "gl/SkMesaGLContext.h"
 #include "gl/GrGLDefines.h"
 
+SkMesaGLContext::AutoContextRestore::AutoContextRestore() {
+    fOldContext = (Context)OSMesaGetCurrentContext();
+    if (fOldContext) {
+        OSMesaGetColorBuffer((OSMesaContext)fOldContext,
+                              &fOldWidth, &fOldHeight,
+                              &fOldFormat, &fOldImage);
+    }
+}
+
+SkMesaGLContext::AutoContextRestore::~AutoContextRestore() {
+    if (fOldContext) {
+        OSMesaMakeCurrent((OSMesaContext)fOldContext, fOldImage,
+                          fOldFormat, fOldWidth, fOldHeight);
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////////
+
 SkMesaGLContext::SkMesaGLContext()
     : fContext(static_cast<Context>(NULL))
     , fImage(NULL) {