Only count markers that are part of the selected range when hovering a marker in the marker chart.

[fqueze]

Fixes a bug in the implementation of #4892.

Steps to reproduce:

• Load this example profile: https://share.firefox.dev/3wMQqtN
• Hover a marker and see the marker count
• Change the preview selection
• See that less markers are drawn
• Hover a marker. Notice the marker count is still the same as before; it should be lower.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.46%. Comparing base (ffe2b6a) to head (1a18805).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5004 +/- ## ======================================= Coverage 88.46% 88.46% ======================================= Files 304 304 Lines 27317 27329 +12 Branches 7377 7378 +1 ======================================= + Hits 24165 24177 +12 Misses 2930 2930 Partials 222 222 ```

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[fqueze]

Thanks for the review!

I'm not super happy about this duplication, but let's do that...

I'm not super happy about the duplication either. I initially wanted to make a shared method returning timeAtViewportLeft and timeAtViewportRightPlusMargin, but then I realized some of the intermediary variables are used for the other caller, so I wouldn't actually win much.