DevTools: split network project into buckets

DevTools: split network project into buckets

Currently, for a given target and frame, NetworkProject puts all
UISourceCodes in a single project. This is true for UISourceCodes
being created for scripts, styles, resources and source maps.

In future, however, we'd like to have all the UISourceCodes in a
separate projects, which are fully managed by responsible bindings.
(design doc: https://goo.gl/rt9RAS).

This patch prepares for the step, splitting NetworkProject into
multiple "buckets":
- ResourcesBucket: contains UISourceCodes for resourceTreeModel
  resources
- ScriptsBucket: contains UISourceCodes for scripts
- StyleSheetsBucket: contains UISourceCodes for style sheets
- SourceMapBucket: contains UISourceCodes for source map sources

Today, the buckets are cleaned up altogether in
frameDetached/mainFrameNavigated events.

Our goal is to clean up buckets independently.

BUG=670180
R=dgozman

[lushnikov, 2017-03-09 00:38:58]

please, take a look

[lushnikov, 2017-03-09 00:39:05]

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

[commit-bot: I haz the power, 2017-03-09 00:39:31]

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

[lushnikov, 2017-03-09 15:46:21]

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

[commit-bot: I haz the power, 2017-03-09 15:46:37]

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 16:40:13]

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

[commit-bot: I haz the power, 2017-03-09 16:40:14]

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

[dgozman, 2017-03-10 01:51:27]

The code lgtm, but let's do more patches before landing this to be sure it all
works out.

[lushnikov, 2017-04-04 20:53:50]

I don't think we need this patch any more. It's main point was to prepare for
dependency removal between debugger/css/sourcemap UISourceCodes and
ResourceTreeModel.

The dependency is fully removed in https://codereview.chromium.org/2795193003/#,
so closing this.