gn win: Various toolchain fixes to get compile working

gn win: Various toolchain fixes to get compile working

This fixes
"fatal error C1902: Program database manager mismatch; please check your installation"
and
"cannot find msvcr120.dll"
during gn compilation.

There were many things wrong. The primary ones were that:
1) the compiler's setup script wasn't called so we didn't get
   PATH/LIB/INCLUDE for the correct cl.
2) the VS runtime dirs were not getting added to environment block.

I also removed the Express handling, as that's been removed from the
gyp path now.

This does not yet support a system-installed Visual Studio. Code
needs to be added to query the registry to find the install location.
(see _SetupScript).

R=dpranke@chromium.org
TBR=brettw@chromium.org
BUG=432375
Committed: https://crrev.com/9bf8fb3b74de68a85122cb45bb477d604b8c3898
Cr-Commit-Position: refs/heads/master@{#304864}

[scottmg, 2014-11-19 18:30:13]

scottmg@chromium.org changed reviewers:
+ brettw@chromium.org

[Dirk Pranke, 2014-11-19 18:44:44]

lgtm. Thanks for working on this! It would've taken me a lot longer ...

[scottmg, 2014-11-19 18:49:17]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2014-11-19 18:50:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/736073002/20001

[commit-bot: I haz the power, 2014-11-19 19:33:37]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-11-19 19:34:19]

Patchset 2 (id:??) landed as
https://crrev.com/9bf8fb3b74de68a85122cb45bb477d604b8c3898
Cr-Commit-Position: refs/heads/master@{#304864}