Fix MB configurations for Mac and Win GN bots.

Fix MB configurations for Mac and Win GN bots.

The windows bots were incorrectly doing 64-bit builds, and the
debug bots (both mac and win) were incorrectly doing shared builds.

TBR=phajdan.jr@chromium.org
BUG=454413
CQ_EXTRA_TRYBOTS=tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Committed: https://crrev.com/31a66f3e705d20a39033a7b7d7f09a60517e6b9d
Cr-Commit-Position: refs/heads/master@{#325297}

[Dirk Pranke, 2015-04-15 20:11:58]

The CQ bit was checked by dpranke@chromium.org

[Dirk Pranke, 2015-04-15 20:12:08]

The CQ bit was unchecked by dpranke@chromium.org

[Dirk Pranke, 2015-04-15 20:12:50]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-04-15 20:15:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1086163002/1

[commit-bot: I haz the power, 2015-04-15 20:51:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-15 20:52:05]

Patchset 1 (id:??) landed as
https://crrev.com/31a66f3e705d20a39033a7b7d7f09a60517e6b9d
Cr-Commit-Position: refs/heads/master@{#325297}