Make SkGLContext lifetime more well-defined

Make SkGLContext lifetime more well-defined

Remove refcounting from SkGLContext.

SkGLContext is expected to behave like GrContextFactory would own
it, as implied by the GrContextFactory function.

If it is refcounted, this does not hold.

Also other use sites, such as in SkOSWindow_win (command buffer gl
object), confirm the behavior. The object is explicitly owned and
destroyed, not shared.

Also fixes potential crashes from using GL context of an abandoned
context.

Also fixes potential crashes in DM/nanobench, if the GrContext lives
longer than GLContext through internal refing of GrContext.

Moves the non-trivial implementations from GrContextFactory.h to
.cpp, just for consistency sake.

Changes pathops_unittest.gyp. The pathops_unittest uses
GrContextFactory, but did not link to its implementation. The reason
they worked was that the implementation used (constructors, destructors)
happened to be in the .h file.

This works towards being able to use command buffer and NVPR from
the SampleApp.

BUG=skia:2992
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Review URL: https://codereview.chromium.org/1511773005