Update Trace Collection UI.

Screencast: https://screencast.googleplex.com/cast/NTg5MjQwMDk0NDc3NTE2OHxmNTAzNTQ2NC1mMw

Reflect tracing start latency to avoid the script crashing/no files being loaded due to tracing not having begun before the user presses "End trace".
Unify use of load progress component to ensure consistency with loading icons and other css.

Fixes: 315976233
Test: npm run test:unit:ci
Change-Id: I7976d94cf43a265056c014f6d16b1b1665036b9f
6 files changed