Improve image perf debugging experience on Memory page

[cobblest]

Based on UI perf diary homework study, participants noted there were several things they would like to improve on the Memory page to better help them debug image perf issues

• Many used the Memory tab to solve Task 4 (Image issue) and debug other image related performance issues. It was hard to find image details information (e.g. unclear how Int64List was related to images information).
• Some terms, icons, and legends were difficult to understand for new users. They need more intuitive legend and documentation.
• They also wanted to better connect event (Timeline) and Memory.
• Autosnapshot was helpful and some warnings would be useful too.

We could fix this by

• Open legend by default on the right side of memory event chart.
• Add tooltips and links to documentation on Memory page.
• Help users find image details information (size, names, etc.)
• Warn users when there are big memory issues/large images.

[srawlins]

Closing as obsolete: This feedback from 2021 pertained to the legacy Memory screen and auto-snapshotting UI, which has since been completely redesigned with new Diff, Profile, and Tracing views.

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