Hold a reference to ShaderDiskCache while its Init() is called

ShaderDiskCache's reference count used to be 0 at the beginning of Init(), and be incremented
implicitly by base::Bind there. As the reference count is decremented to 0 if CreateCacheBackend
finished immediately and the passed callback is destroyed.
That causes unexpected destruction of the SharedDiskCache instance.

After this CL, the caller of Init() keeps an reference while Init() is running to avoid
destroying the ShaderDiskCache instance.

Bug: 866456
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: Iddf9daaf09119aed279ffb81f0af1a74efb9dc54
Reviewed-on: https://chromium-review.googlesource.com/1148090Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577613}

gpu/ipc/host/shader_disk_cache.cc

@@ -468,7 +468,7 @@ scoped_refptr<ShaderDiskCache> ShaderCacheFactory::GetByPath(
   if (iter != shader_cache_map_.end())
     return iter->second;
 
-  ShaderDiskCache* cache = new ShaderDiskCache(this, path);
+  auto cache = base::WrapRefCounted(new ShaderDiskCache(this, path));
   cache->Init();
   return cache;
 }