base: Use fdatasync() on Fuchsia.

This CL switches base::File::Flush() from fsync() to fdatasync() on
Fuchsia. This matches the behavior on all other POSIX platforms that
implement fdatasync().

Change-Id: I69406ff5091fca626004370d70b5530c4a23f2c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394722
Commit-Queue: Victor Costan <pwnall@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813567}

base/files/file_posix.cc

@@ -549,7 +549,8 @@ bool File::Flush() {
 #if defined(OS_NACL)
   NOTIMPLEMENTED();  // NaCl doesn't implement fsync.
   return true;
-#elif defined(OS_LINUX) || defined(OS_CHROMEOS) || defined(OS_ANDROID)
+#elif defined(OS_ANDROID) || defined(OS_CHROMEOS) || defined(OS_FUCHSIA) || \
+    defined(OS_LINUX)
   return !HANDLE_EINTR(fdatasync(file_.get()));
 #elif defined(OS_APPLE)
   // On macOS and iOS, fsync() is guaranteed to send the file's data to the