Enable GPU Rasterization on Imagination on Chrome OS

Like the subject says, since this passes skia gold tests we are
enabling GPU rasterization on Imagination GPUs.

BUG=b:65493006
TEST=build on elm, see GPU rasterization enabled

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: I78e015241d12ad8c7804737c972667026b815394
Reviewed-on: https://chromium-review.googlesource.com/1229034Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Stéphane Marchesin <marcheu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592163}

gpu/config/software_rendering_list.json

@@ -1362,7 +1362,7 @@
     },
     {
       "id": 137,
-      "description": "GPU rasterization on CrOS is blacklisted on anything but Intel or Mali GPUs for now.",
+      "description": "GPU rasterization on CrOS is blacklisted on anything but Intel, Mali or Imagination GPUs for now.",
       "cr_bugs": [684094],
       "os": {
         "type": "chromeos"
@@ -1372,7 +1372,8 @@
       ],
       "exceptions": [
         { "vendor_id": "0x8086" },
-        { "gl_renderer": "Mali-T.*" }
+        { "gl_renderer": "Mali-T.*" },
+        { "gl_renderer": "PowerVR.*" }
       ]
     },
     {