Booking.com's anti-bot script pre-defines a non-configurable own `src` on
<img> elements. The Image shim's load emulation then threw "TypeError:
Cannot redefine property: src" out of the constructor, crashing any page
that calls new Image() after that instrumentation runs.
Check the own descriptor first and skip the load emulation when `src` is
already non-configurable: a page that owns `src` is instrumenting loads
itself. The normal path (configurable src, emulated load event) is
unchanged and now covered by a test.
Fixes the crash half of #394.
Jim Zhang @ wonderhoi
891d850d19