Changes the set counting to use a free count by descriptor type. Also
changes the init to take a pool size count that more closely mirrors
Vulkan.

Will lead to a fix for descriptor set recreation.

Bug: angleproject:2678
Change-Id: I26816befec28ebb866fb0f3aaba27d2bc1d3be43
Reviewed-on: https://chromium-review.googlesource.com/1110661Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>