android: Trim instead of reset program cache on background

Deleting the program cache is not safe because not all channels are
closed before hand. There's specific check for leaving webgl context
behind. Those context would be using a dangling program cache pointer
after that.

Bug: 756497
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I28b23f66049a92dae9fe4490f0850d2207f07f9f
Reviewed-on: https://chromium-review.googlesource.com/646238
Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#499112}

gpu/ipc/service/gpu_channel_manager.cc

@@ -307,7 +307,8 @@ void GpuChannelManager::OnApplicationBackgrounded() {
   for (int channel : channels_to_clear)
     RemoveChannel(channel);
 
-  program_cache_.reset();
+  if (program_cache_)
+    program_cache_->Trim(0u);
 }
 #endif