Use PartitionAlloc for blink::PolicyContainer

In a previous CL, I forgot to add USING_FAST_MALLOC to
the class blink::PolicyContainer which is supposed to
be owned by a unique_ptr.

Bug: 1130587
Change-Id: Ie608376c763cab4770e72e00c7fac984c81f89b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526303Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826213}

third_party/blink/renderer/core/frame/policy_container.h

@@ -21,6 +21,8 @@ namespace blink {
 // parses meta tags). The corresponding setters trigger also an update in the
 // corresponding PolicyContainerHost via a mojo IPC.
 class CORE_EXPORT PolicyContainer {
+  USING_FAST_MALLOC(PolicyContainer);
+
  public:
   PolicyContainer() = delete;
   PolicyContainer(