resourceloader: Remove mwPerformance stub and rename mwLoadStart mark

resourceloader: Remove mwPerformance stub and rename mwLoadStart mark

Remove the private 'mwPerformance' stub, in favour of simply calling
performance.mark() directly in a conditional, like we do elsewhere for
web APIs that don't exist in all supported browsers.

This mark (part of the W3C User Timing interface), was sometimes used
to measure a delta to mwLoadEnd, whcih is a problem because startup
loads asynchronously. The point it executes is intentionally variable.
What matters is when the overall JS load process ends, which is composed
of multiple requests.

Keep the mark, but rename to 'mwStartup' which better describes its
purpose and can remain useful.

Bug: T160315
Bug: T192623
Change-Id: I6e2049e7252e5c2be3c5bc6617703b62e5edafb4