Restrict disable_gl_rgb_format to Linux only

This workaround was added for Linux set-top boxes with Mali-400.
However, the workaround is not needed on Android devices with Mali-400
(probably due to using a completely different driver).

The disable_gl_rgb_format code path is poorly tested and had at least
one bug, so reduce the workaround to only apply to Linux.

Bug: 1018528, 449150
Change-Id: Ic2f5248a8ff943edab90c24577ea94601693a32e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891139Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711006}

gpu/config/gpu_driver_bug_list.json

@@ -904,7 +904,10 @@
     {
       "id": 108,
       "cr_bugs": [449150],
-      "description": "Mali-4xx does not support GL_RGB format",
+      "description": "Mali-4xx on Linux does not support GL_RGB format",
+      "os": {
+        "type": "linux"
+      },
       "gl_vendor": "ARM.*",
       "gl_renderer": ".*Mali-4.*",
       "features": [