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It appears that this fixes the issue where rendered contents wouldn't
appear on the screen. I say "it appears" because this bug is very
difficult to repro, and so I can never say it's actually gone, just
that I wasn't able to make it happen anymore.

I can come up with a line of reasoning that the glBegin/End causes
the driver to go and validate its internal state. Perhaps sometimes the
dirty bit for the FBO is missed (and doing the glBegin/End immediately
after changing the FBO ensure it's picked up). Ultimately it is just a
guess.

This workaround was discovered by accident through the following
sequence of steps when debugging:
1. Draw a triangle to the CAOpenGLLayer after drawing the texture, to
   ensure that we are actually getting draw calls
   - This appeared and kept updating even when the texture stopped
     updating
2. Draw a triangle to the FBO's texture from the CAOpenGLLayer's
   context, to make sure changes to the texture would go through
   - This appeared and kept updating even when the textures topped
     updating (so it's probably a problem with the command buffer
     context).
3. Draw a triangle to the FBO's texture from the command buffer's
   context just before glSwapBuffers
   - This triangle never appeared, so I tried the next experiment.
4. Draw a triangle to the FBO's texture right after it is bound using
   glBindFramebufferEXT
   - Suddenly the bug went away (and I never saw the triangle, because
     it was drawn over).
5. Just do a glBegin/End with program 0, since we found this was enough to
   work around driver bugs in the past.
   - Still couldn't repro the bug with this.

BUG=435786

Review URL: https://codereview.chromium.org/797533002

Cr-Commit-Position: refs/heads/master@{#308166}