Fix allocation of context buffers on frame resize

The patch:
https://gerrit.chromium.org/gerrit/#/c/70814/
changed the test that determined whether the context
frame buffers needed to be reallocated or not.

The code checked for a change in total frame area
to signal the need to reallocate context buffers.
However, the above_context buffer needs to be
resized i:xf only the width of the frame has increased.

Change-Id: Ib89d75651af252908144cf662578d84f16cf30e6