This moves ValidateDrawBase into a common inline function. It also
moves some checks in ValidateDrawElementsCommon into
ValidateDrawElementsBase. "Base" is called from DrawElementsIndirect
while "Common" is only called from non-indirect entry points. But this
improves conformance because the missing checks in "Base" apply to
DrawIndirect as well.

In a follow-up patch many checks in "Base" will be cached into a single
value. Much like what we did for ValidateDrawBase.

Also inlines ValidateDrawElements which just calls through to "Common".

Bug: angleproject:2966
Change-Id: Ibe0c5db25f403dd52a50dc73373ebb50cedad003
Reviewed-on: https://chromium-review.googlesource.com/c/1357147
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>