*re-land with build fix for Clang*

We had a funny bug where the Handle Allocator would re-allocate
reserved handles after the app layer creates one with Bind rather
than using Gen. This affects Textures, Buffers and Renderbuffers.

Fix this by using a different allocation scheme. It should still
be fast on the "good" case (using Gen) and use tree lookups on the
bind case. Also add some unit tests.

BUG=angleproject:942

Change-Id: I63ce608fcd6a11f92e2b5421f090551934e729ed
Reviewed-on: https://chromium-review.googlesource.com/261591Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>