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This puts the gles2 thunks target in a more reasonable location, stops abusing
static_library to hide dependency issues, and stop the view manager client side
library from pretending to know which sort of gles2 to link against. It uses
the gles2 headers but depending on whether the lib is being linked into an
application or linked into a target that links in the shell lib (as is done in
unit tests) it needs a different implementation. This also deletes the unused
//mojo/public/gles2/gles2_interface.h file - if we want this we can pull it
back but since nobody is using it now it's kind of a waste of space. If we do
want it, it should go in //mojo/public/cpp/gles2 since it's a C++ wrapper.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/604223002

Cr-Commit-Position: refs/heads/master@{#297040}