Speed up mergetraces.py.

Speed up mergetraces.py.

list -> set, O(n^2) -> O(n).
Timing, without this patch (on a typical trace from a device):
real	1m49.774s
user	1m49.394s
sys	0m0.316s

With:
real	0m5.607s
user	0m5.482s
sys	0m0.120s

BUG=448054
Committed: https://crrev.com/e45ccdcce7af6e91ca25f0660c1339b044c9ed3a
Cr-Commit-Position: refs/heads/master@{#313513}

[Benoit L, 2015-01-28 14:36:16]

lizeb@chromium.org changed reviewers:
+ pasko@chromium.org

[pasko, 2015-01-28 14:38:25]

oops, good catch, lgtm++ !!

[Benoit L, 2015-01-28 14:38:53]

The CQ bit was checked by lizeb@chromium.org

[commit-bot: I haz the power, 2015-01-28 14:39:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/873003003/1

[commit-bot: I haz the power, 2015-01-28 15:14:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-28 15:16:11]

Patchset 1 (id:??) landed as
https://crrev.com/e45ccdcce7af6e91ca25f0660c1339b044c9ed3a
Cr-Commit-Position: refs/heads/master@{#313513}