media/base: Update WritableUnalignedMapping constructor.

Use the base::UnsafeSharedMemoryRegion constructor which avoids a mojo
dependency. Several media tests are built without mojo support.

Bug: 795291
Change-Id: If754e6111189e8944997550f00e3ed7ef9fde1c3
Reviewed-on: https://chromium-review.googlesource.com/1190243Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588808}

media/base/unaligned_shared_memory.cc

@@ -113,10 +113,7 @@ WritableUnalignedMapping::WritableUnalignedMapping(
     size_t size,
     off_t offset)
     : WritableUnalignedMapping(
-          mojo::UnwrapUnsafeSharedMemoryRegion(mojo::WrapSharedMemoryHandle(
-              handle,
-              handle.GetSize(),
-              mojo::UnwrappedSharedMemoryHandleProtection::kReadWrite)),
+          base::UnsafeSharedMemoryRegion::CreateFromHandle(handle),
           size,
           offset) {}