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java_cpp_enum.gypi was creating all enums in the same root folder and
then adding that folder to its dependents generated_src_dirs. This has
two issues: first, incremental builds will include stale files when
things are moved/renamed/etc. second, all libraries that depend on such
an enum target will actually compile and include all the enum targets
(and in fact may even include different versions of one enum in
 different libraries).

This change just makes each such target use its own unique directory
(this will still have the stale enum issue when renaming an enum if the
 target name doesn't change, but that is very rare).

The GN version already used unique directories.

TBR=mkosiba

Review URL: https://codereview.chromium.org/578363002

Cr-Commit-Position: refs/heads/master@{#295595}