[vm] Don't enable the profiler by default. Enable the profiler at startup with --observe, or later …

runtime/vm/profiler.cc

Index: runtime/vm/profiler.cc
diff --git a/runtime/vm/profiler.cc b/runtime/vm/profiler.cc
index a99b48ee84b38090e31fdc1e74dc7b6e2eef88a3..50eab00071fbf7d11ecd6ab109415e9d9859aa4d 100644
--- a/runtime/vm/profiler.cc
+++ b/runtime/vm/profiler.cc
@@ -78,6 +78,7 @@ void Profiler::InitOnce() {
   // Zero counters.
   memset(&counters_, 0, sizeof(counters_));
   NativeSymbolResolver::InitOnce();
+  ThreadInterrupter::InitOnce();
   ThreadInterrupter::SetInterruptPeriod(FLAG_profile_period);
   ThreadInterrupter::Startup();
   initialized_ = true;
@@ -97,6 +98,8 @@ void Profiler::Shutdown() {
   ASSERT(initialized_);
   ThreadInterrupter::Shutdown();
   NativeSymbolResolver::ShutdownOnce();
+  // Note we do not free the sample buffer because there may be SIGPROFs
+  // in flight.
 }
 
 void Profiler::SetSampleDepth(intptr_t depth) {