Reduce flakiness of import-profiler presubmit #18987

In PR #18986, the import-profiler presubmit failed with:

FAILURE: Import time regression of 244.54 ms exceeds both the absolute threshold (100.0 ms) and the relative threshold (201.76 ms, 15% of baseline Median).

See build log.

I inspected the PR, but couldn't spot any obvious regression. The surface of the API has increased, so the increase in baseline is expected. This presubmit should not fail for routine changes. I'm going to propose to increase the baseline diff threshold from 100ms to 300ms, but we should investigate if there are other improvements that we can make in the generated library, and restore the original 100 ms.