Otherwise, end2end tests crash on Nexus 5X,
because Adreno driver doesn't provide glMapBuffer.

BUG=angleproject:1362
TEST=angle_end2end_tests on Nexus 5X and Linux

Change-Id: I6207eca2d0e03eee957bb612271a66016fd562bb
Reviewed-on: https://chromium-review.googlesource.com/349421Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>