Move skia command buffer bindings to gpu/skia_bindings

The skia GrGLInterface <-> gpu command buffer bindings are needed by anything
that wants to use skia's GL backend with chromium's GL stack. This includes
the compositor. As such, these bindings should live somewhere that the compositor
can depend on without depending on webkit/ things. They can't live directly in
gpu/command_buffer/ as that can't depend on skia, and they can't live in
skia/ as gpu/ currently depends on skia/ through ui/gfx. Thus they live in
a new directory gpu/skia_bindings/ which depends on other things in gpu/ but is
not depended on by anything in gpu/, much like gpu/ipc.

BUG=181120

Review URL: https://codereview.chromium.org/100563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240086 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240086