Add GLSL/ESSL validator/translator support for GL_EXT_shader_framebuffer_fetch.

BUG=angle:834

Change-Id: I2d4e25909a8e1266b9bb7f1d4421324143157c8a
Reviewed-on: https://chromium-review.googlesource.com/231032
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Erik Dahlström <ed@opera.com>
Remove RemoveAllTreeNodes, since it was a no-op

IntermNode operator delete() or any of the IntermNode destructors don't
do anything, since all the AST memory is allocated on the PoolAllocator.
Because of this, RemoveAllTreeNodes was simply a no-op, and redundant
with the PoolAllocator deallocation procedure, and could confuse people
reading the code to think that IntermNodes should be deleted
individually, when in fact this is not the case.

BUG=angle:831

Change-Id: Ie1ccaa51986aabf267280d92a8e76ca9f97a19e5
Reviewed-on: https://chromium-review.googlesource.com/230730
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>