GN: Add --depfile to generate_incremental_manifest.py action.

GN: Add --depfile to generate_incremental_manifest.py action.

Fixes the target always being considered dirty

BUG=

Committed: https://crrev.com/5a3bf6ce59ee0d9b37c935d7166aacb9feec8cc9
Cr-Commit-Position: refs/heads/master@{#352399}

[agrieve, 2015-10-05 14:47:02]

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

[agrieve, 2015-10-05 14:47:15]

The CQ bit was checked by agrieve@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-05 14:47:34]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1389663002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1389663002/1

[commit-bot: I haz the power, 2015-10-05 14:55:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-05 14:55:57]

Dry run: Try jobs failed on following builders:
  linux_chromium_clobber_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[agrieve, 2015-10-05 15:01:35]

On 2015/10/05 14:55:57, commit-bot: I haz the power wrote:
> Dry run: Try jobs failed on following builders:
>   linux_chromium_clobber_rel_ng on tryserver.chromium.linux (JOB_FAILED,
>
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

ptAl

[pkotwicz, 2015-10-05 19:48:14]

lgtm

[agrieve, 2015-10-05 19:51:23]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-10-05 19:54:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1389663002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1389663002/1

[commit-bot: I haz the power, 2015-10-05 20:13:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-05 20:15:02]

Patchset 1 (id:??) landed as
https://crrev.com/5a3bf6ce59ee0d9b37c935d7166aacb9feec8cc9
Cr-Commit-Position: refs/heads/master@{#352399}