Tell MSAN that profiler doesn't expect sp to point at initialized memory

BUG=v8:4444,v8:4151
LOG=N

Review URL: https://codereview.chromium.org/1371253002

Cr-Commit-Position: refs/heads/master@{#30996}

src/profiler/sampler.cc

@@ -682,6 +682,9 @@ DISABLE_ASAN void TickSample::Init(Isolate* isolate,
     external_callback = scope->callback();
     has_external_callback = true;
   } else {
+    // sp register may point at an arbitrary place in memory, make
+    // sure MSAN doesn't complain about it.
+    MSAN_MEMORY_IS_INITIALIZED(regs.sp, sizeof(Address));
     // Sample potential return address value for frameless invocation of
     // stubs (we'll figure out later, if this value makes sense).
     tos = Memory::Address_at(reinterpret_cast<Address>(regs.sp));