Make ExtensionEventHistogramValueTest handle multiline event entries

This CL changes the ExtensionEventHistogramValueTest implementation
such that it can also handle multiline entries in
extensions/browser/extension_event_histogram_value.h . With the current
implementation a multiline entry (e.g. due to a long event name)
results in an unexpected test error that is hard to understand.

Change-Id: Id2495912c055b46811f00bf767a2ad39932928d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5079783
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#1233879}