tools/gn: make defined(foo.bar) an error if !defined(foo)

tools/gn: make defined(foo.bar) an error if !defined(foo)

Users must now write "defined(foo) && defined(foo.bar)" if they want
the old semantics.  No Chromium BUILD.gn files were harmed in the
making of this CL.

Committed: https://crrev.com/365796868448a3983a5831abdc37f67588390dde
Cr-Commit-Position: refs/heads/master@{#321808}

[mdempsky, 2015-03-21 23:51:53]

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

[brettw, 2015-03-23 17:22:56]

lgtm

[mdempsky, 2015-03-23 17:27:46]

The CQ bit was checked by mdempsky@chromium.org

[commit-bot: I haz the power, 2015-03-23 17:27:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1025153002/1

[commit-bot: I haz the power, 2015-03-23 18:14:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-23 18:15:17]

Patchset 1 (id:??) landed as
https://crrev.com/365796868448a3983a5831abdc37f67588390dde
Cr-Commit-Position: refs/heads/master@{#321808}