Change FilePath to use EnableIf instead of native typemapping.

Bug: 676224
Change-Id: I101d138ac8ca72815cc6aa0a1b5ad4218b3c3001
Reviewed-on: https://chromium-review.googlesource.com/1065753
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560839}

• chrome/services/util_win/public/mojom/shell_util_win.typemap[diff]
• mojo/public/cpp/base/BUILD.gn[diff]
• mojo/public/cpp/base/file_path.typemap[diff]
• mojo/public/cpp/base/file_path_mojom_traits.cc[Added - diff]
• mojo/public/cpp/base/file_path_mojom_traits.h[Added - diff]
• mojo/public/cpp/base/file_path_unittest.cc[diff]
• mojo/public/mojom/base/BUILD.gn[diff]
• mojo/public/mojom/base/file_path.mojom[diff]
• ui/display/mojo/display_snapshot_struct_traits.cc[diff]
• ui/display/mojo/display_struct_traits_unittest.cc[diff]