[heap profiler] Fix the unsupported stack mode warning.

Fixes the following warning message when --memlog is used.
"ERROR:settings.cc(119)] Unsupported value: "" passed to --memlog-stack-mode"

Change-Id: I33d80a29ea117777e935199f1ab83827c7b3fbb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1586539Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654660}

components/services/heap_profiling/public/cpp/settings.cc

@@ -102,6 +102,8 @@ mojom::StackMode GetStackModeForStartup() {
   } else {
     stack_mode = base::GetFieldTrialParamValueByFeature(
         kOOPHeapProfilingFeature, kOOPHeapProfilingFeatureStackMode);
+    if (stack_mode.empty())
+      stack_mode = kMemlogStackModeNative;
   }
 
   return ConvertStringToStackMode(stack_mode);