Free fewer resources when idle in oop raster

freeGpuResources evicts harder to regenerate Skia caches, so use the
slightly lighter weight purgeUnlockedResources when idle for oop raster.
This matches the behavior of gpu raster (see: ContextCacheController).

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: Ia954629b13f8d5fcb50cbd314976e8558b1609d4
Reviewed-on: https://chromium-review.googlesource.com/1141102Reviewed-by: Eric Karl <ericrk@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576180}

gpu/command_buffer/service/gr_cache_controller.cc

@@ -46,10 +46,9 @@ void GrCacheController::PurgeGrCache(uint64_t idle_id) {
     return;
   }
 
-  // TODO(khushalsagar): Replace with purgeUnlockedResources which retains the
-  // text cache.
   context_state_->context->MakeCurrent(context_state_->surface.get());
-  context_state_->gr_context->freeGpuResources();
+  context_state_->gr_context->purgeUnlockedResources(
+      false /* scratchResourcesOnly */);
 }
 
 }  // namespace raster