Consistently refcount SiteInstance in the public content API.
In the past, there have been bugs of this nature:
1. Create a SiteInstance and save it in a raw pointer on the stack.
Refcounted objects in Chromium start with a refcount of zero.
2. The SiteInstance is passed to some function that puts it in a
stack scoped_refptr. Ref count is incremented one, function does
some work, and then returns.
3. Stack scoped_refptr decrements the ref count to zero on and the
SiteInstance is freed. Oops.
BUG=none
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation
Review URL: https://codereview.chromium.org/1836423002
Cr-Commit-Position: refs/heads/master@{#383997}
38 files changed
