[heap] Add prefix for young generation handling use counter

R=ulan@chromium.org
BUG=chromium:693413

Change-Id: Ie8e6a5e20e904b81228b67f33c976569e305872c
Reviewed-on: https://chromium-review.googlesource.com/445157Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43319}

src/counters.h

@@ -941,7 +941,7 @@ class RuntimeCallTimerScope {
   HR(incremental_marking_reason, V8.GCIncrementalMarkingReason, 0, 21, 22)    \
   HR(mark_compact_reason, V8.GCMarkCompactReason, 0, 21, 22)                  \
   HR(scavenge_reason, V8.GCScavengeReason, 0, 21, 22)                         \
-  HR(young_generation_handling, V8.YoungGenerationHandling, 0, 1, 2)          \
+  HR(young_generation_handling, V8.GCYoungGenerationHandling, 0, 1, 2)        \
   /* Asm/Wasm. */                                                             \
   HR(wasm_functions_per_module, V8.WasmFunctionsPerModule, 1, 10000, 51)