Fix dynamic freeze scope artificially extended warnings

[metajack]

These are now appearing in servo and a few of the submodules. They should be cleaned up.

See rust-lang/rust#6248 for more information about the warning.

/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:168:34: 168:44 warning: Dynamic freeze scope artifically extended (see Issue #6248)
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:168         let font_ptr: *mut Font = &mut *font;
                                                                                                              ^~~~~~~~~~
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:168:34: 168:44 note: managed value only needs to be frozen for the expression at 168:34...
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:168         let font_ptr: *mut Font = &mut *font;
                                                                                                              ^~~~~~~~~~
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:167:42: 196:5 note: ...but due to Issue #6248, it will be frozen for the block at 167:42
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:167     pub fn new(font: @mut Font) -> Shaper {
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:168         let font_ptr: *mut Font = &mut *font;
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:169         let hb_face: *hb_face_t = hb_face_create_for_tables(get_font_table_func,
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:170                                                             font_ptr as *c_void,
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:171                                                             null());
/Users/jack/src/servo/src/components/servo-gfx/text/shaping/harfbuzz.rs:172         let hb_font: *hb_font_t = hb_font_create(hb_face);
                                                                            ...

[kmcallister]

I think these were gone at one point, but this one appeared recently:

src/components/script/script_task.rs:611:33: 611:43 warning: Dynamic freeze scope artifically extended (see Issue #6248)
src/components/script/script_task.rs:611         let window = Window::new(&mut *page, self.chan.clone(), self.compositor);
                                                                          ^~~~~~~~~~
src/components/script/script_task.rs:611:33: 611:43 note: managed value only needs to be frozen for the expression at 611:33...
src/components/script/script_task.rs:611         let window = Window::new(&mut *page, self.chan.clone(), self.compositor);
                                                                          ^~~~~~~~~~
src/components/script/script_task.rs:611:21: 611:81 note: ...but due to Issue #6248, it will be frozen for the call at 611:21
src/components/script/script_task.rs:611         let window = Window::new(&mut *page, self.chan.clone(), self.compositor);
                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~