quadVertices.data() pointer needs to remain valid until drawArrays
is called. To guarantee that quadVertices should stay in the stack
until the end of drawQuad().

BUG=angleproject:2533
TEST=angle_end2end_tests

Change-Id: I263262db9a9ed9c9cd0d8679addd09677ad7179e
Reviewed-on: https://chromium-review.googlesource.com/1046600
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>