Remove ~1750 superfluous includes from core

Remove ~1750 superfluous includes from core

This patch removes a large number of unnecessary includes from core/.
This only modifies .cpp files, cleaning includes from .h files will
require more work.

BUG=

[eseidel, 2013-05-20 08:27:57]

Are you using a script to find these?  Is there a meta-bug for this work?

[Steve Block, 2013-05-20 08:48:34]

Presumably the motivation for this is to reduce build times?

If so, does this mean that it's our policy to only use the minimum number of
includes required to allow the code to build, even if some required symbols are
picked up from tortuous chains of includes that are likely to change in the
future? Sometimes I add extra includes, even when not strictly required, to
avoid breakages due to changes in such include chains, and to improve
readability. It would be good to know the policy here.

[alph, 2013-05-20 15:48:32]

Timothy,
Do you have a speedup data?
Most modern compilers support include guard optimization, so they won't even
read a header file with a guard past its very first include occurrence.

[pdr., 2013-05-20 17:13:11]

On 2013/05/20 15:48:32, alph wrote:
> Timothy,
> Do you have a speedup data?
> Most modern compilers support include guard optimization, so they won't even
> read a header file with a guard past its very first include occurrence.

I've created a master bug for tracking this work: crbug.com/242216
Can we take the meta discussion there?

[esprehn, 2013-05-20 18:09:00]

How did you come to the conclusion that these includes were not needed? We don't
want to depend on transitive includes everywhere, it makes things fragile.
Ideally we want to go in the other direction with include-what-you-use where you
either include it directly or forward declare it, and don't depend on transitive
inclusion.