Refactor IsolateHolder to be able to always create the isolate

Refactor IsolateHolder to be able to always create the isolate

Currently, blink creates its own isolates. We want to always use an
IsolateHolder to create the isolates. To be able to do this, I introduce
an Initialize method that setups V8. The new IsolateHolder ctor now
doesn't take any parameters but just creates new isolates according to
the configuration.

All non-blink gin users are cut over to the new API

BUG=none
R=abarth@chromium.org,andrewhayden@chromium.org,eroman@chromium.org

Committed: https://crrev.com/2f43f2c92879b36d1262d02fba4e32890912b0b5
Cr-Commit-Position: refs/heads/master@{#294262}

[Andrew Hayden (chromium.org), 2014-09-09 10:08:47]

LGTM, but I'm not familiar with the whole background of gin. Read: looks
syntactically valid and sane, no comment on the improvement itself.

[eroman, 2014-09-10 01:40:06]

net/proxy LGTM

[jochen (gone - plz use gerrit), 2014-09-10 09:50:42]

jochen@chromium.org changed reviewers:
+ aa@chromium.org

[jochen (gone - plz use gerrit), 2014-09-10 09:51:00]

Aaron, could you please have a look at //mojo and //gin

[Aaron Boodman, 2014-09-10 22:09:19]

lgtm

[jochen (gone - plz use gerrit), 2014-09-10 22:10:14]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2014-09-10 22:15:56]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jochen@chromium.org/553903003/20001

[commit-bot: I haz the power, 2014-09-10 23:49:07]

Committed patchset #2 (id:20001) as 11e62ffd667aba41e0cd114760acb532bc62b0cb

[commit-bot: I haz the power, 2014-09-11 00:05:20]

Patchset 2 (id:??) landed as
https://crrev.com/2f43f2c92879b36d1262d02fba4e32890912b0b5
Cr-Commit-Position: refs/heads/master@{#294262}