Implement client side PBOs for glReadPixel

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index ec5b2abd19fde36ab46ebf8054cd5a048e70db81..545c4c77b74c6fc6e19753fcc603805a7fccbb17 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -6947,11 +6947,17 @@ error::Error GLES2DecoderImpl::HandleReadPixels(
   }
   void* pixels = GetSharedMemoryAs<void*>(
       c.pixels_shm_id, c.pixels_shm_offset, pixels_size);
-  Result* result = GetSharedMemoryAs<Result*>(
-        c.result_shm_id, c.result_shm_offset, sizeof(*result));
-  if (!pixels || !result) {
+  if (!pixels) {
     return error::kOutOfBounds;
   }
+  Result* result = NULL;
+  if (c.result_shm_id != 0) {
+    result = GetSharedMemoryAs<Result*>(
+        c.result_shm_id, c.result_shm_offset, sizeof(*result));
+    if (!result) {
+      return error::kOutOfBounds;
+    }
+  }
 
   if (!validators_->read_pixel_format.IsValid(format)) {
     LOCAL_SET_GL_ERROR_INVALID_ENUM("glReadPixels", format, "format");
@@ -7031,7 +7037,9 @@ error::Error GLES2DecoderImpl::HandleReadPixels(
   }
   GLenum error = LOCAL_PEEK_GL_ERROR("glReadPixels");
   if (error == GL_NO_ERROR) {
-    *result = true;
+    if (result != NULL) {
+      *result = true;
+    }
 
     GLenum read_format = GetBoundReadFrameBufferInternalFormat();
     uint32 channels_exist = GLES2Util::GetChannelsForFormat(read_format);