ASSERT memcache.c:332 !exists || vmvector_overlap(all_memory_areas, start, end) || are_dynamo_vm_areas_stale()

ASSERT memcache.c:332 !exists

Running Chromium base_unittests on ARM we hit:

```
[ RUN ] MemoryMappedFileTest.MapPartialRegionAtEnd

Note: Google Test filter = MemoryMappedFileTest.*
[==========] Running 8 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 8 tests from MemoryMappedFileTest
[ RUN ] MemoryMappedFileTest.MapWholeFileByPath
[ OK ] MemoryMappedFileTest.MapWholeFileByPath (35 ms)
[ RUN ] MemoryMappedFileTest.MapWholeFileByFD
[ OK ] MemoryMappedFileTest.MapWholeFileByFD (10 ms)
[ RUN ] MemoryMappedFileTest.MapSmallFile
[ OK ] MemoryMappedFileTest.MapSmallFile (8 ms)
[ RUN ] MemoryMappedFileTest.MapWholeFileUsingRegion
[ OK ] MemoryMappedFileTest.MapWholeFileUsingRegion (10 ms)
[ RUN ] MemoryMappedFileTest.MapPartialRegionAtBeginning
[ OK ] MemoryMappedFileTest.MapPartialRegionAtBeginning (11 ms)
[ RUN ] MemoryMappedFileTest.MapPartialRegionAtEnd