Add missing (and remove superfluous) 'explicit' from constructors.

Add missing (and remove superfluous) 'explicit' from constructors.

This is part of a clean-up in preparation for having the Clang style-check
plugin checking for this automatically.

BUG=163357

[hans, 2012-11-29 16:08:35]

Apologies for the mega-patch, but this is 99% mechanical. The classes that
relied on having an implicit constructor are:

IPC::ChannelHandle, net::uint128 and cc::Region, so please give them extra
attention.

mark: can you do OWNERS for base/ ?
enne: can you do OWNERS for cc/ ?
jochen: can you do OWNERS for chrome/ ?
piman: can you do OWNERS for content/ ?
apatrick: can you do OWNERS for gpu/ ?
zea: can you do OWNERS for jingle/ ?
szym: can you do OWNERS for net/ ?
simonmorris: can you do OWNERS for remoting/ ?
sky: can you do OWNERS for ui/ ?
tony: can you do OWNERS for base/ ?
wtc: can you do OWNERS for base/ ?
satorux: can you do OWNERS for dbus/ ?
jeremy: can you do OWNERS for ipc/ ?
vandebo: can you do OWNERS for skia/ ?
tim: can you do OWNERS for sync/ ?

[Mark Mentovai, 2012-11-29 16:44:39]

You’ve chosen 15 reviewers for a mechanical cleanup change. This is a spam trap.
No.

Either choose one competent reviewer for the whole thing, or break it up into 15
separate changes so as to not spam the world over this.

[hans, 2012-11-29 17:22:12]

On 2012/11/29 16:44:39, Mark Mentovai wrote:
> You’ve chosen 15 reviewers for a mechanical cleanup change. This is a spam
trap.
> No.
> 
> Either choose one competent reviewer for the whole thing, or break it up into
15
> separate changes so as to not spam the world over this.

OK. Removing everyone from this until I've figured out how to get it reviewed.
Sorry for the spam.