EyeDropper API: repeated open/select should not freeze the sampler

Manual regression test for crbug.com/509627745. On macOS Tahoe, repeatedly opening and selecting from the eye dropper could freeze the sampler (spinning wait cursor, stuck-open picker). This test asks for 10 round trips. PASS if all 20 selections complete without the sampler hanging.

TODO: Click on the eye dropper icon, then click on the red canvas. Repeat 20 times.