What goes inside and outside a render pass command buffer is largely
mutually exclusive.  Moreover, the size and frequency of allocations is
different between the two.

This change distinguishes the C++ types used for inside and outside
render pass command buffers:

- The type now documents which command buffer a function is able to
  receive.
- `isRenderPass` flag passing, checking and asserting is largely
  removed.
- A follow up change experiments with using different (Vulkan vs ANGLE)
  secondary command buffers for inside and outside RP command buffers.
- A future change could specialize the pool behaviors per command buffer
  type.

Bug: angleproject:6811
Change-Id: Ia4bc669d26ac7e94e8a0dfb9b361666c82f42cc3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3344373


Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>