Fuchsia: enable MemoryMappedFile tests

Fuchsia: enable MemoryMappedFile tests
Fuchsia now supports mmap() for tmpfs, so these tests Just Work.


Bug: 746051, 706592
Change-Id: I208c88852bcd22cec0b03f8c2578c71fd305c663
Reviewed-on: https://chromium-review.googlesource.com/596611Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491205}

testing/buildbot/filters/fuchsia.base_unittests.filter

@@ -25,16 +25,6 @@
 -FileUtilProxyTest.Touch
 -FileUtilTest.FileToFILE
 -LoggingTest.CheckCausesDistinctBreakpoints
--MemoryMappedFileTest.ExtendableFile
--MemoryMappedFileTest.MapLargePartialRegionInTheMiddle
--MemoryMappedFileTest.MapPartialRegionAtBeginning
--MemoryMappedFileTest.MapPartialRegionAtEnd
--MemoryMappedFileTest.MapSmallFile
--MemoryMappedFileTest.MapSmallPartialRegionInTheMiddle
--MemoryMappedFileTest.MapWholeFileByFD
--MemoryMappedFileTest.MapWholeFileByPath
--MemoryMappedFileTest.MapWholeFileUsingRegion
--MemoryMappedFileTest.WriteableFile
 -NativeLibraryTest.LoadLibrary
 -NativeLibraryTest.LoadLibraryPreferOwnSymbols
 -PartitionAllocTest.Basic