base: Remove default mode from base::File::Lock().

The distinction between kShared and kExplicit is large enough that there
are significant readability advantages to having the default be explicit
in the code.

Bug: 1125315
Change-Id: I965c1c04eaa3dbd26191847f800adbbf7a0cf35f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394198Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806478}

base/files/file.h

@@ -299,7 +299,7 @@ class BASE_EXPORT File {
   //  * Within a process, locking the same file (by the same or new handle)
   //    will succeed. The new lock replaces the old lock.
   //  * Closing any descriptor on a given file releases the lock.
-  Error Lock(LockMode mode = LockMode::kExclusive);
+  Error Lock(LockMode mode);
 
   // Unlock a file previously locked.
   Error Unlock();