Add support for additional tree map analysis types to Heap Snapshot

[bkonyi]

The DevTools heap snapshot viewer should support additional tree map based analysis options:

• Dominator analysis
• Merged dominator analysis

cc @rmacnak-google

[srawlins]

I believe this is obsolete: The legacy Heap Snapshot treemap and analysis view has been removed as part of the Memory screen redesign (which now centers around Diff Snapshots, Profile Memory, and Trace Instances).

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!