Use unique_ptr<> for BrowserPluginGuest creation APIs

Bug: 419087
Change-Id: Ic870c7059fef752560d0299064a0cef4a1733f3a
Reviewed-on: https://chromium-review.googlesource.com/c/1277594
Commit-Queue: Albert J. Wong <[email protected]>
Reviewed-by: Nasko Oskov <[email protected]>
Reviewed-by: danakj <[email protected]>
Reviewed-by: James MacLean <[email protected]>
Cr-Commit-Position: refs/heads/master@{#599077}
4 files changed