Restrict disable_discard_framebuffer to only Mali on ChromeOS

The problem was only observed on Mali and could even be fixed in recent
driver versions. For now, at least narrow down the workaround to only
apply to Mali GPUs. On other GPUs we observe around 10% improvement in
framerate, which we can't leave on the table.

Bug: 301988
Change-Id: Iab07a60a74555d6935b53dc30af1b798a1c56e8f
Reviewed-on: https://chromium-review.googlesource.com/c/1312158Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Kristian H. Kristensen <hoegsberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605049}

gpu/config/gpu_driver_bug_list.json

@@ -320,6 +320,8 @@
       "os": {
         "type": "chromeos"
       },
+      "gl_vendor": "ARM.*",
+      "gl_renderer": "Mali.*",
       "features": [
         "disable_discard_framebuffer"
       ]