fix incorrect line chart size in FireFox
Created by: unclejustin
Swap getBoundingClientRect for getBBox. There are known differences in how FireFox calculated bounding rectangle width using getBoundingClientRect. Switching to getBBox offers similiar enough functionality for our use case as well as consistent results across browsers.
Test plan
Open any dashboard with insights Resize the screen Charts should resize proportionally across all browsers.
Closes https://github.com/sourcegraph/sourcegraph/issues/34668
App preview:
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