Add base:: qualification to callback examples.

Code was difficult to copy-and-paste out of the callback examples
because typically one would need to add base:: to many types.

This explicitly adds "base::" to all examples.

Originally I was going to keep the references in the text unchanged and
only change the code blocks. But this ended up being confusing. As a
result, all code-formatted references have been qualified. In some cases
this is a bit verbose and repetitive, but I think its better to be
explicit and consistent.

Expands a bit on weak pointer usage which I have found myself repeatedly
looking up.

Change-Id: I961b25ea6db6151180cf0d687e576be72e327427
Reviewed-by: Albert J. Wong <>
Commit-Queue: Brett Wilson <>
Cr-Commit-Position: refs/heads/master@{#504781}
1 file changed