RFR(S) 8077400: Unnecessary and incorrect "Code Cache Roots" G1 log entry

[Mikael Gerdin]

Hi all,

When "JDK-8027962 Per-phase timing measurements for strong roots 
processing" was ported to the G1RootProcessor change it still contained 
a left over reference to "Code Cache Roots". Code Cache scanning is 
already done and measured lower in the call chain in G1.
The current "Code Cache Roots" measurement is actually the sum of the 
UpdateRS, ScanRS and Code Root Scan times.

I also took the liberty to remove the comment about the complement of 
the collection set, since it should be more clear now that the call to 
oops_into_collection_set_do is in a separate function with a fairly 
descriptive name.

I also updated the TestGCLogMessages test to not expect the Code Cache 
Roots log entry.

Bug: https://bugs.openjdk.java.net/browse/JDK-8077400
Webrev: http://cr.openjdk.java.net/~mgerdin/8077400/webrev.0

Testing: JPRT

/Mikael