Add instructions for checking out OpenJDK ToT.

This CL adds the instructions to CompareUpstreams.java, just because
similar instructions were already there (note that no file in Android
is based on OpenJDK head / ToT, so strictly speaking it isn't an upstream).

Test: Treehugger

Change-Id: I2c19967bb39cae538fee5f1f78fc0b31b05cca57