GN: Fix clean build of cloud_policy_proto_generated_compile_proto failing

GN: Fix clean build of cloud_policy_proto_generated_compile_proto failing

Failed due to forward_variables_from("deps") clobbering existing deps,
and ./protoc not being built before invoking it.

Commit that introduced this bug:
https://crrev.com/a09029884e02a3c027a485877886cf75969f6b3d

BUG=

Committed: https://crrev.com/4fb6b6f46c01262f37c7a170f3de2366f4d8cedf
Cr-Commit-Position: refs/heads/master@{#342205}

[agrieve, 2015-08-06 18:37:31]

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

[agrieve, 2015-08-06 18:37:32]

Seems like this is also indicative of a bug in GN, since the docs say:

When an explicit list of variables is supplied, if the variable exists in the
current (destination) scope already, an error will be thrown.

Yet no error was thrown during gn gen.

[brettw, 2015-08-06 19:04:47]

lgtm

[agrieve, 2015-08-06 19:58:48]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-08-06 19:59:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1278793003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1278793003/1

[commit-bot: I haz the power, 2015-08-06 21:42:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-06 21:42:36]

Patchset 1 (id:??) landed as
https://crrev.com/4fb6b6f46c01262f37c7a170f3de2366f4d8cedf
Cr-Commit-Position: refs/heads/master@{#342205}