[import] Let imported documents visible in DevTools resource panel.

This change tells DevTools about Resources and their type of HTML Imports.
The change:

- maps Resouce::ImportResource to InspectorPageAgent::DocumentResource.
- traverses import tree to return all resources in the tree for InspectorPageAgent::getResourceTree()
- Let HTMLImportChild retain Resource object for imports even loading is finished so that getResourceTree can find it.

TEST=resource-tree-htmlimports.html
BUG=334388
R=pfeldman, vsevik, dglazkov

Review URL: https://codereview.chromium.org/141653007

git-svn-id: svn://svn.chromium.org/blink/trunk@165605 bbb929c8-8fbe-4397-9dbb-9b2b20218538