Only commit cookie changes in prerenders after a prerender is shown

Only commit cookie changes in prerenders after a prerender is shown
Will create a PrerenderCookieStore for each prerender, retaining all cookie
operations of a prerender until the prerender is shown to the user.
Forces prerenders to be in a new render process by themselves for this to work.
This is a resubmission of https://codereview.chromium.org/233353003, which had
to be reverted due to build breaks. See LGTM's there.
BUG=371003
TBR=jam@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270049

[tburkard, 2014-05-12 22:21:40]

The CQ bit was checked by tburkard@chromium.org

[commit-bot: I haz the power, 2014-05-12 22:22:44]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tburkard@chromium.org/280403002/20001

[commit-bot: I haz the power, 2014-05-12 22:22:45]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-12 22:22:45]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[tburkard, 2014-05-13 05:32:57]

The CQ bit was checked by tburkard@chromium.org

[commit-bot: I haz the power, 2014-05-13 05:33:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tburkard@chromium.org/280403002/40001

[tburkard, 2014-05-13 05:50:09]

The CQ bit was checked by tburkard@chromium.org

[commit-bot: I haz the power, 2014-05-13 05:50:34]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tburkard@chromium.org/280403002/60001

[commit-bot: I haz the power, 2014-05-13 08:20:06]

Change committed as 270049