6919980: G1: remove +UseG1GC from under experimental options (second attempt)
Summary: Trying this again, as the original change was lost.
Reviewed-by: ysr, jmasa
^build/
^dist/
/nbproject/private/
^src/share/tools/hsdis/build/
^src/share/tools/IdealGraphVisualizer/[a-zA-Z0-9]*/build/
^src/share/tools/IdealGraphVisualizer/build/
^src/share/tools/IdealGraphVisualizer/dist/