Add hook for sdkextras, clean out stale entries

Add hook for sdkextras, clean out stale entries

We need to run the sdkextras hook to download the google play services
jar on the buildbots. Developers should run
build/install-build-deps-android.sh manually to review the license terms.

Also removes some stale hooks:
- install-arm-sysroot does nothing
- we don't need a win_toolchain as we don't build on windows.

R=alhaad@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/138597c0ab2cdbcee0e43dd14c2d313171f20c6e

[jamesr, 2015-02-18 23:38:31]

jamesr@chromium.org changed reviewers:
+ alhaad@chromium.org
- alhaad@google.com

[alhaad, 2015-02-19 00:09:00]

lgtm.

[jamesr, 2015-02-19 00:10:24]

Committed patchset #1 (id:1) manually as
138597c0ab2cdbcee0e43dd14c2d313171f20c6e (presubmit successful).