When glBufferData is called on a mapped buffer, per the OpenGL ES 3.0
spec it is implicitly unmapped. Later calls to glMapBufferRange should
not throw an error.

This CL unmaps the buffer in BufferData if it is already mapped.
Also adds a new test that verfies the behavior.

Test: angle_end2end_tests --gtest_filter=BufferDataTestES3.BufferDataUnmap/*
Test: Angry Birds 2 MEC
Bug: angleproject:4599
Bug: b/157672184
Change-Id: I1a1e458aa5f50da4dfde9f6847f71cd5b6f6c08a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2233365
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Manh Nguyen <nguyenmh@google.com>