Allow trace-viewer to use exec_script in GN.

Allow trace-viewer to use exec_script in GN.

Trace-viewer currently maintains two copies of its input files. This
causes duplicate work each time we add a file. I'd like to share a
single .gypi file between GYP and GN but need to use exec_script
in order to do so.

This CL adds an exeception to allow trace-viewer to use exec_script
in its BUILD.gn file.

Committed: https://crrev.com/f4e1a7ade87e16677474ff662bffb03ae0ae0047
Cr-Commit-Position: refs/heads/master@{#332892}

[dsinclair, 2015-06-04 14:39:24]

dsinclair@chromium.org changed reviewers:
+ brettw@chromium.org, dpranke@chromium.org

[dsinclair, 2015-06-04 14:39:24]

PTAL.

[brettw, 2015-06-04 19:18:55]

lgtm

[dsinclair, 2015-06-04 19:29:41]

The CQ bit was checked by dsinclair@chromium.org

[commit-bot: I haz the power, 2015-06-04 19:30:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1160793005/1

[commit-bot: I haz the power, 2015-06-04 20:05:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-04 20:06:41]

Patchset 1 (id:??) landed as
https://crrev.com/f4e1a7ade87e16677474ff662bffb03ae0ae0047
Cr-Commit-Position: refs/heads/master@{#332892}