Previously transient CommandPool is used for CommandBuffer allocation,
it is created and destroyed per frame. However, profiling found that
CommandPool destroy is very inefficient. So this commit removed the
previous logic and use two preallocated resetable CommandPools (One for
Primary and One for Secondary)

Bug: angleproject:3508
Change-Id: I8b36f2738b082811c3177935c61b10e01acb6947
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1648667
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>