Remove needless '&mut self' from HTMLDivElementMethods.
Remove needless '&mut self' from HTMLDListElementMethods.
Remove needless '&mut self' from HTMLEmbedElementMethods.
Remove needless '&mut self' from HTMLFieldSetElementMethods.
Remove needless '&mut self' from HTMLFontElementMethods.
Remove needless '&mut self' from HTMLFormElementMethods.
Remove needless '&mut self' from HTMLFrameElementMethods.
Remove needless '&mut self' from HTMLFrameSetElementMethods.
Remove needless '&mut self' from HTMLHeadingElementMethods.
Remove needless '&mut self' from HTMLHRElementMethods.
Remove needless '&mut self' from HTMLLegendElementMethods.
Remove needless '&mut self' from HTMLLabelElementMethods.
Remove needless '&mut self' from HTMLInputElementMethods.
Remove needless '&mut self' from HTMLHtmlElementMethods.
Remove needless '&mut self' from HTMLLIElementMethods.
Remove needless '&mut self' from HTMLLinkElementMethods.
auto merge of #2520 : saneyuki/servo/mut, r=Ms2ger

Related #2514

- This removes needless `&mut self`s from some DOM type method simply.
- This doesn't include some DOM Types which have basis method called from other types (e.g. `Element`). We still have to need to open their changes.
- This does not include to remove an implicit `deref_mut()`.
Fix minor misformatting in codegen.
Prevent child layers from overriding root background color

The first layer implicitly provides the size of the page, but child
layer background colors can still improperly override the body
background color. This commit ensures that layer background colors only
apply to layers with the same id and pipeline id. Additionally the root
layer's unrendered color is defined by the first layer's background
color, just like for size.
auto merge of #2522 : Ms2ger/servo/codegen-cleanup, r=jdm