* Took IconsProcessor out of ManifestProcessor.
* Converted IconsProcessor into ImageObjectProcessor.
* Made extractValue a separate resource, as it's used by multiple
processors.
* Taught manifest processor about splash_screens.
* icons member now uses ImageObjectProcessor.
* Icon tests are now ImageProcessor tests.
* Fixed a few typos.
--HG--
rename : dom/manifest/test/test_IconsProcessor_density.html => dom/manifest/test/test_ImageObjectProcessor_density.html
rename : dom/manifest/test/test_IconsProcessor_sizes.html => dom/manifest/test/test_ImageObjectProcessor_sizes.html
rename : dom/manifest/test/test_IconsProcessor_src.html => dom/manifest/test/test_ImageObjectProcessor_src.html
rename : dom/manifest/test/test_IconsProcessor_type.html => dom/manifest/test/test_ImageObjectProcessor_type.html
extra : rebase_source : f2f6ad690a9c5b15193fd3567122a47a23ff07bc
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