Keep the PlatformDelegate in a std::unique_ptr instead of a scoped_refptr
in FilePathWatcher. Use weak ptrs when posting callbacks bound to a
PlatformDelegate.

With this change, it is guaranteed that a PlatformDelegate does not
outlive its parent FilePathWatcher. This will allow multiple code
simplifications in FilePathWatcher.

BUG=675631

Review-Url: https://codereview.chromium.org/2596273003
Cr-Commit-Position: refs/heads/master@{#442404}