Rename existing depthTexture extension flag to depthTextureANGLE to
distinguish it from OES depth texture extension to be added.

Bug: angleproject:3103
Test: angle_end2end_tests --gtest_filter=DepthStencilFormatsTest.DepthTexture
Change-Id: I44c69a69b925a84f931518e0374e662893813061
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1575425
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>