Separate draw call validation functions will give us a flexible
base for refactoring and cleaning up all draw call validation.

BUG=angle:571

Change-Id: Ia8e3c83cfe48fedb8f2c7aef1fb282f646c66e82
Reviewed-on: https://chromium-review.googlesource.com/202974Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>