WebGL can request contexts with a stencil buffer but no depth buffer.

BUG=angleproject:2075

Change-Id: I46b272089e44d5f1725fa6252b68d34c9e9f37f6
Reviewed-on: https://chromium-review.googlesource.com/565052Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>