Add mean GC throughput as per CPU time to perfdump

Add mean GC throughput as per CPU time to the GC performance dump as
is already done in case of individual collectors.

Test: art/test/testrunner/testrunner.py --host --runtime-option=-XX:DumpGCPerformanceOnShutdown
Bug: 130362501
Change-Id: I0af44ba73a54de3e7490b09f3c0eddd26df519a5
1 file changed