Web Replay: fix spammy function call tracing when not capturing or replaying.
Web Replay: Only call function tracing instrumentation in InspectorReplayAgent in debug builds.
Web Replay: clean up header includes and guards in NetworkProxy.
Web Replay: improve error diagnostics in NetworkProxy for missing or mismatched resource requests.
Web Replay: memoize document.lastModified.

The last modified value for a document is nondeterministically generated from the current system time
if no Last-Modified header was found for the original resource (the common case).