Speed up "gn path".

Speed up "gn path".

"gn path" finds all unique paths. This can be slow because, for example,
//chrome/browser has 478 million unique paths through it's dependency tree
and running path from //chrome/browser to //chrome/renderer took about 6.5
seconds on my big desktop to analyze all of these (to find no results).

This patch keeps track of which targets have been eliminated from consideration
and avoids visiting them again. This improves the above qwuery to 0.78 seconds
while still being able to compute the total number of results.

Committed: https://crrev.com/22ca12c760e23efdc988409149fd2d4e43e6fed9
Cr-Commit-Position: refs/heads/master@{#330772}

[brettw, 2015-05-20 16:45:37]

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

[brettw, 2015-05-20 16:45:42]

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

[commit-bot: I haz the power, 2015-05-20 16:46:12]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1143043004/1

[commit-bot: I haz the power, 2015-05-20 17:00:06]

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

[commit-bot: I haz the power, 2015-05-20 17:00:06]

Dry run: This issue passed the CQ dry run.

[Dirk Pranke, 2015-05-20 17:33:58]

lgtm

[brettw, 2015-05-20 18:07:35]

The CQ bit was checked by brettw@chromium.org

[commit-bot: I haz the power, 2015-05-20 18:08:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1143043004/1

[commit-bot: I haz the power, 2015-05-20 18:12:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-20 18:13:52]

Patchset 1 (id:??) landed as
https://crrev.com/22ca12c760e23efdc988409149fd2d4e43e6fed9
Cr-Commit-Position: refs/heads/master@{#330772}