export re2::FilteredRE2::* for chromium

patch from gentoo
export only specific symbol, matching more makes unit tests to abort

obj/dbg/test/mimics_pcre_test           FAIL; output in obj/so/test/filtered_re2_test.log

re2/prefilter_tree.cc:288: Compile() not called
re2/testing/filtered_re2_test.cc:35: Check failed: (0) == (v.atoms.size())
FilteredRE2Test.EmptyTest
FilteredRE2Test.SmallOrTest
Aborted

$ gcc --version
gcc (PLD-Linux) 4.7.2 20121104 (release)
add  doc/syntax.txt to doc