Purge UnspecifiedBoolType from Blink.

The modern pattern is "explicit operator bool". std::unique_ptr and WTF::RefPtr
already do this, so this just pushes out to the remaining uses, except for one
in WTF::String, which exists to conflict with operator NSString* on Mac and
prevent boolean conversion via that (implicitly).

Review-Url: https://codereview.chromium.org/2145643002
Cr-Commit-Position: refs/heads/master@{#405013}
5 files changed