In Context::onDestroy(), e7b3fe21 had
moved surface deletion first which down the line caused
RendererVk::finish() to be called.

bf7b95db however made surface deletion
unnecessary, which means finish was never called.

This commit adds an explicit finish in Context::onDestroy().  In truth,
the wait is only necessary until all command buffers submitted for this
particular context have finished.  This optimization is deferred to a
possible future work.

Bug: angleproject:2811
Change-Id: I56e6c88d3b4a6ec73f70d80d7775a0c85be651ea
Reviewed-on: https://chromium-review.googlesource.com/c/1302838
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>