[debugger] Re-enable --always-opt in one debugger test.

R=yangguo@chromium.org
TEST=cctest/test-debug/Backtrace

Review URL: https://codereview.chromium.org/1415463017

Cr-Commit-Position: refs/heads/master@{#31746}

test/cctest/test-debug.cc

@@ -6706,10 +6706,6 @@ TEST(Backtrace) {
 
   v8::Debug::SetMessageHandler(BacktraceData::MessageHandler);
 
-  // TODO(mstarzinger): This doesn't work with --always-opt because we don't
-  // have correct source positions in optimized code. Enable once we have.
-  i::FLAG_always_opt = false;
-
   const int kBufferSize = 1000;
   uint16_t buffer[kBufferSize];
   const char* scripts_command =