Bug 1754277: Fix non-unified build errors in layout/svg, and mark it as safe to build in non-unified mode. r=emilio

Below are the build errors fixed here.

layout/svg/SVGFEUnstyledLeafFrame.cpp:75:33: error: no type named 'SVGFEUnstyledElement' in namespace 'mozilla::dom'

* Fixed this by including SVGFilters.h (which defines this type).

layout/svg/SVGFEContainerFrame.cpp:83:12: error: unknown type name 'SVGFE'; did you mean 'dom::SVGFE'?
layout/svg/SVGFELeafFrame.cpp:80:12: error: unknown type name 'SVGFE'; did you mean 'dom::SVGFE'?

* Fixed these with a 'using' statement for the namespace.

layout/svg/SVGGFrame.cpp:34:32: error: unknown type name 'SVGElement'

* Fixed by including this type's header.

layout/svg/FilterInstance.cpp:873:9: error: use of undeclared identifier 'nsLayoutUtils'
layout/svg/SVGOuterSVGFrame.cpp:623:29: error: incomplete type 'nsLayoutUtils' named in nested name specifier
layout/svg/SVGUseFrame.cpp:84:5: error: use of undeclared identifier 'nsLayoutUtils
layout/svg/SVGViewportFrame.cpp:207:9: error: use of undeclared identifier 'nsLayoutUtils'

* Fixed these by including this header.

layout/svg/SVGOuterSVGFrame.cpp:88:20: error: unused function 'DependsOnIntrinsicSize' [-Werror,-Wunused-function]

* Fixed by removing the unused function.

layout/svg/SVGStopFrame.cpp:86:21: error: unknown type name 'SVGGradientFrame'

* Fixed this by including this header.

Differential Revision: https://phabricator.services.mozilla.com/D138187