[wasm] Disable V8.WasmExecutionTimeMicroseconds timing

This timer imposes a high overhead and does not give us the data we'd
like. Disabling for now until we can develop a better solution.

Bug: v8:6514
Change-Id: I73b15131a71d7b6750556f82907cb2a0e6edd321
Reviewed-on: https://chromium-review.googlesource.com/633703
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47582}

src/isolate.cc

@@ -1225,7 +1225,6 @@ Object* Isolate::UnwindAndFindHandler() {
         }
 
         if (!FLAG_experimental_wasm_eh || !is_catchable_by_wasm(exception)) {
-          counters()->wasm_execution_time()->Stop();
           break;
         }
         int stack_slots = 0;  // Will contain stack slot count of frame.

src/runtime/runtime-wasm.cc

@@ -197,17 +197,11 @@ RUNTIME_FUNCTION(Runtime_WasmSetCaughtExceptionValue) {
 
 RUNTIME_FUNCTION(Runtime_SetThreadInWasm) {
   trap_handler::SetThreadInWasm();
-  if (!isolate->counters()->wasm_execution_time()->Running()) {
-    isolate->counters()->wasm_execution_time()->Start();
-  }
   return isolate->heap()->undefined_value();
 }
 
 RUNTIME_FUNCTION(Runtime_ClearThreadInWasm) {
   trap_handler::ClearThreadInWasm();
-  if (isolate->counters()->wasm_execution_time()->Running()) {
-    isolate->counters()->wasm_execution_time()->Stop();
-  }
   return isolate->heap()->undefined_value();
 }