DevTools: cleanup script UISourceCodes on executionContextDestroyed

DevTools: cleanup script UISourceCodes on executionContextDestroyed

Currently, NetworkProject does a best effort to clean up
UISourceCodes when the frame gets detached. Dynamic scripts, for
example, are not getting removed.

This patch teaches NetworkProject to listen to ExecutionContextDestroyed
event in order to remove associated script-related UISourceCodes.

As a result, this patch un-binds script-related UISourceCode management
from ResourceTreeModel.

R=dgozman
BUG=670180

[lushnikov, 2017-03-09 04:22:40]

please, take a look

[lushnikov, 2017-03-09 04:22:44]

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

[commit-bot: I haz the power, 2017-03-09 04:22:54]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-09 05:41:17]

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

[commit-bot: I haz the power, 2017-03-09 05:41:18]

Dry run: This issue passed the CQ dry run.

[dgozman, 2017-03-10 01:50:36]

lgtm

[lushnikov, 2017-04-04 20:50:55]

The "buckets" change which this relies on doesn't seem to be needed - this patch
is re-implemented in https://codereview.chromium.org/2795193003/.