[handles] Sanitize Handle and friends.

[handles] Sanitize Handle and friends.

Bunch of cleanups to allow us to get rid of handles-inl.h at some
point (in the not so far future); but more importantly to sanitize uses
of handles and prepare for handle canonicalization support.

R=yangguo@chromium.org

Committed: https://crrev.com/3283195d0408333cce552cf4087577e6f41054e5
Cr-Commit-Position: refs/heads/master@{#28222}

Review URL: https://codereview.chromium.org/1128533002

Cr-Commit-Position: refs/heads/master@{#29666}