webview: add trace event for relro timeout.

Record an instant trace event if we encounter a timeout while waiting
for relro creation, using the same message we already log.

We're seeing a number of occurrences of this timeout in cases where
WebView is otherwise working in traces, so this should help to debug.

Using the ACTIVITY_MANAGER trace tag as it's enabled for the trace
collection pipeline already; WEBVIEW is not.

Bug: 235546861
Test: n/a
Change-Id: I2b60c5699efde026da934457e430adaebad1d2b3