- Asynchronously load source map from sourceMappingURL
  - Once loaded, annotate source positions with their original position
  - Update script panel tooltip to include link to original source
     - For the above, make DOM.element a slightly more flexible API,
       allowing defining attributes and children
     - Also fix ToolTipEvent handling to support nodes.
  - Shuffle around some code to make createScriptNode async, in case
    we want to load the source map when building the script node itself.
  - Drive-by: make source markers a simple backgroundColor when there is
    only one group.

Change-Id: I0926807761cbfe8b6dd8ff5154815a7e5ccb39bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2972827
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75302}