[PM] Process Metrics Decorator - Prevent a crash in unit tests

The Memory Instrumentation service isn't available in unit_tests, this
add a check to verify that the service exists before trying to calling
it.

Change-Id: I6f4ceb28fe828b8229888bbff5d58a445051cd5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783304Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694752}

chrome/browser/performance_manager/decorators/process_metrics_decorator.cc

@@ -52,8 +52,12 @@ void ProcessMetricsDecorator::RequestProcessesMemoryMetrics(
         callback) {
   auto* mem_instrumentation =
       memory_instrumentation::MemoryInstrumentation::GetInstance();
-  mem_instrumentation->RequestPrivateMemoryFootprint(base::kNullProcessId,
-                                                     std::move(callback));
+  // The memory instrumentation service is not available in unit tests unless
+  // explicitly created.
+  if (mem_instrumentation) {
+    mem_instrumentation->RequestPrivateMemoryFootprint(base::kNullProcessId,
+                                                       std::move(callback));
+  }
 }
 
 void ProcessMetricsDecorator::DidGetMemoryUsage(