Matches style guide requirement for types before members. Also
moves the implementation of the class entirely into the cpp file.
Moves the method implementation in ContextVk so we can more easily
alter member variables.

Unrelated cleanup done while working on consolidating RenderPasses.

Bug: angleproject:4911
Change-Id: Ibe4273fc609b494840f1e86584bcee5bc31397d5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2331950
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>