Convert coverageMutator to a TransitionMutator

Convert coverageMutator to a TransitionMutator as a step towards
variants-on-demand.

Bug: 319288033
Test: coverage_test.go
Test: treehugger coverage builds
Change-Id: Ic50c0040dea8b42c36b5d784221daa00b7b0d379
diff --git a/cc/cc.go b/cc/cc.go
index c07e358..41fea47 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -68,7 +68,7 @@
 
 		ctx.TopDown("fuzz_deps", fuzzMutatorDeps)
 
-		ctx.BottomUp("coverage", coverageMutator).Parallel()
+		ctx.Transition("coverage", &coverageTransitionMutator{})
 
 		ctx.TopDown("afdo_deps", afdoDepsMutator)
 		ctx.BottomUp("afdo", afdoMutator).Parallel()