By default, the libchromiumcontent build makes all static libraries "thin."  From the `ar` man page:
> GNU ar can optionally create a thin archive, which contains a symbol index and references to the original copies of the member files of the archives.  Such an archive is useful for building libraries for use within a local build, where the relocatable objects are expected to remain available, and copying the contents of each object would only waste time and space.

Thin `.a` files are useless as artifacts, because they only contain references to `.o` files, rather than actual compiled code.  The `standalone_static_library` variable forces a given library to be built as a traditional, standalone `.a` file that can be used by other builds.