Bug 218000

To make things simpler, and allow for passing FEATURE_AND_PLATFORM_DEFINES as a file rather than on the command line, we should remove the in-makefile use of grepping FEATURE_AND_PLATFORM_DEFINES.

For the ones conditionalizing adding to ADDITIONAL_BINDING_IDLS, those IDLs should just have the appropriate Conditional=* extended attribute added, then they can be included unconditionally in the makefile.

For the ones conditionalizing adding go USER_AGENT_STYLE_SHEETS, again, there doesn’t seem a real reason to keep that. The sheets are all preprocessed anyway, so we can just move those #ifdefs into the files.