WebPrivatePtr: avoid creating temporary Persistent<> on assignment.
WebPrivatePtr: avoid creating temporary Persistent<> on assignment.

For RefCountedGarbageCollected objects wrapped in a WebPrivatePtr, the
PtrStorageImpl::assign(PassRefPtr<T>) overload potentially created a
Persistent on assigning its underlying raw pointer, even if a RawPtr<>
was passed along (due to the implicit RawPtr<T> conversion that
PassRefPtr<T> provides.)

This assign() overload serves no real purpose with Oilpan enabled;
provide a more appropriate RawPtr<T> version there.

R=haraken@chromium.org,tkent@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/313173009

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@175558 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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