Timeline: do not imply event.thread.target is the main target

Timeline: do not imply event.thread.target is the main target

We used to rely on event.thread.target in multiple places where
target was needed. These places mostly imply that this was a main
target (i.e. using it to get DOMModel & co), while it can now be a
different (or null) target. Let's keep main target associated with
timeline in Timeline model (and perhaps return null for recorded
timelines later) and plumb it where it is necessary.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=185294

[caseq, 2014-11-12 17:30:03]

caseq@chromium.org changed reviewers:
+ alph@chromium.org, yurys@chromium.org

[alph, 2014-11-13 11:11:40]

lgtm

https://codereview.chromium.org/717243002/diff/20001/Source/devtools/front_en...
File Source/devtools/front_end/timeline/TimelineFrameModel.js (right):

https://codereview.chromium.org/717243002/diff/20001/Source/devtools/front_en...
Source/devtools/front_end/timeline/TimelineFrameModel.js:268: else {
should be on the previous line

[caseq, 2014-11-13 12:58:28]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-11-13 12:58:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/717243002/40001

[caseq, 2014-11-13 13:28:06]

The CQ bit was unchecked by caseq@chromium.org

[caseq, 2014-11-13 14:02:57]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2014-11-13 14:03:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/717243002/60001

[commit-bot: I haz the power, 2014-11-13 15:05:47]

Committed patchset #4 (id:60001) as 185294