Use base::File to store breadcrumbs

This change replaces the MemoryMappedFile used by breadcrumbs with a
regular File. MemoryMappedFile adds complexity, and likely doesn't
doesn't provide any performance benefit because:
1. it's currently re-initialized on every call to DoWriteEventsToFile
2. the file is small, only around 3 KB

I think this change will also reduce the incidence of
hard-to-understand crash signatures, e.g.,
https://b.corp.google.com/issues/328945927.
I believe this is actually a disk error caused by faulty hardware,
appearing as a memory error (SIGBUS) because MemoryMappedFile
reports disk errors that way. For more detailed discussion, see
https://issues.chromium.org/issues/40249770#comment4.

Bug: 328945927, 369460150
Change-Id: I14ccab48316ec927e99ff0f82738ba44aef02a57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6056067
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1391322}