Implement the deference operator for scoped_refptr.

scoped_refptr previously got this for free since it implemented operator
T*, but the conversion operator is being removed. This prevents awkward
looking code like *my_favorite_pointer.get().

BUG=110610

Review URL: https://codereview.chromium.org/505943002

Cr-Commit-Position: refs/heads/master@{#291869}
1 file changed