RFR: JDK-8187543 Replace SRC_ROOT with TOPDIR

[Magnus Ihse Bursie]

Due to historical reasons, we have had an "alias" SRC_ROOT that contains 
the same value as TOPDIR. We should remove the former and replace it 
with the latter.

The intention is to push this to the consolidated forest, once it opens.
Bug: https://bugs.openjdk.java.net/browse/JDK-8187543
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8187543-replace-SRC_ROOT-with-TOPDIR/webrev.01

/Magnus