Serializes vertex arrays' states stored on CPU.

Adds vertex array serialization to serializeContext method so that
capture replay regresssion testing now compares the states of vertex
array objects too.

Bug: angleproject:4817
Change-Id: Ia2897c056adff9bd433765186240ab07f78db232
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2331737
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>