Work around linker error of multiple definitions of pp::BlockUntilComplete

ppapi/cpp/completion_callback.h

Index: ppapi/cpp/completion_callback.h
diff --git a/ppapi/cpp/completion_callback.h b/ppapi/cpp/completion_callback.h
index 9539dfc73b659a8b3c0879cde99da36b248659cf..2391a3a5e1b506e61ba69d3a4ff801ff545cf267 100644
--- a/ppapi/cpp/completion_callback.h
+++ b/ppapi/cpp/completion_callback.h
@@ -255,7 +255,11 @@ class CompletionCallbackWithOutput : public CompletionCallback {
 /// allowed from background threads.
 ///
 /// @return A <code>CompletionCallback</code> corresponding to a NULL callback.
-CompletionCallback BlockUntilComplete();
+inline CompletionCallback BlockUntilComplete() {
+  // Note: Explicitly inlined to avoid link errors when included into
+  // ppapi_proxy and ppapi_cpp_objects.
+  return CompletionCallback();
+}
 
 }  // namespace pp