Various test fixtures have acquired assertions and expectations on pointers which try to compare explicitly with NULL or nullptr, rather than testing pointers as bools. These cases will break when WeakPtr is fixed to provide explicit operator bool().

Some bool-checks of other pointer containers (e.g. foo.get() == NULL) are also cleaned up.

BUG=605794

Review-Url: https://codereview.chromium.org/2007373004
Cr-Commit-Position: refs/heads/master@{#396270}