RFR: 6766: Add heap usage before and after GC to the Garbage Collections table [v3]

[Jean-Philippe Bempel]

> Columns are hidden for now
> 
> Here some screenshots of the changes:
> 
> ![JMC_UsedHeapBeforeAfterGC](https://user-images.githubusercontent.com/4610701/94465934-f030a300-01c0-11eb-8492-2b921f695833.png)
> 
> 
> 
> ![JMC_UsedHeapBeforeAfterGCColumns](https://user-images.githubusercontent.com/4610701/94465962-fc1c6500-01c0-11eb-8dc9-5b3346668381.png)

Jean-Philippe Bempel has updated the pull request incrementally with one additional commit since the last revision:

  fix constant name for columns

-------------

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/112/files
  - new: https://git.openjdk.java.net/jmc/pull/112/files/d03089d6..6c82cdbc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=112&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=112&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/112.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/112/head:pull/112

PR: https://git.openjdk.java.net/jmc/pull/112