Track clear counts in vulkan render passes

Track clear counts in vulkan render passes

Though not required by spec, the validation layers have started spitting
out warnings if the clearValueCount in BeginRenderPass is greater than
the number of attachments actually being cleared. This just adds tracking
of the count to appease the validation layers.

BUG=skia:

Change-Id: Iac6500df3ed5ad3f5df5f045c6e533bb021857aa
Reviewed-on: https://skia-review.googlesource.com/7401
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>