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So I tested on a laptop with Win10, with Intel/Radeon HD 6490, the videos listed in  crbug.com/451420  as not working are all working fine even with enabled D3D11.

I'll re-enable D3D11 and WebGL2 on AMD switchable on Win 10.

It seems kbr already enabled it on drivers newer than 2016.6 in  crbug.com/755722 

The driver I tested is older than that, and it's working fine. So I guess likely all drivers on Win10 is OK.

Ken, do you think it's worth enabling older AMD switchable drivers on Win10? 

Sure, go ahead from my standpoint.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dedd64d752ea72dbfea25bfdfb3f47c6e995a1b9

commit dedd64d752ea72dbfea25bfdfb3f47c6e995a1b9
Author: Zhenyao Mo <zmo@chromium.org>
Date: Thu Jun 07 00:24:41 2018

Enable D3D11 for AMD switchable on Win10

Regardless of driver being older

BUG= 721121 
TEST=manual on a AMD switchable win10 device with older driver
R=kbr@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I25456570984ccbe2bcdf0d452acb71e37a8571ca
Reviewed-on: https://chromium-review.googlesource.com/1089835
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565118}
[modify] https://crrev.com/dedd64d752ea72dbfea25bfdfb3f47c6e995a1b9/gpu/config/gpu_driver_bug_list.json
[modify] https://crrev.com/dedd64d752ea72dbfea25bfdfb3f47c6e995a1b9/gpu/config/software_rendering_list.json