Add 'build' directory to import path of gyp_chromium.

Add 'build' directory to import path of gyp_chromium.

Change https://codereview.chromium.org/1418513007/ moved a module
from tools/ to build/ causing gyp_chromium to import a different
module of the same name and breaking because the function names
are different.

BUG=

Committed: https://crrev.com/d9aaae0056107fc54f7d068b8baa8648b6615054
Cr-Commit-Position: refs/heads/master@{#356842}

[bcwhite, 2015-10-29 14:11:58]

bcwhite@chromium.org changed reviewers:
+ jochen@chromium.org, mcgrathr@chromium.org

[jochen (gone - plz use gerrit), 2015-10-29 14:37:43]

lgtm

[bcwhite, 2015-10-29 14:55:16]

The CQ bit was checked by bcwhite@chromium.org

[commit-bot: I haz the power, 2015-10-29 14:55:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1416223004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1416223004/1

[commit-bot: I haz the power, 2015-10-29 15:08:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-29 15:09:12]

Patchset 1 (id:??) landed as
https://crrev.com/d9aaae0056107fc54f7d068b8baa8648b6615054
Cr-Commit-Position: refs/heads/master@{#356842}