remove Isolate::debugger_initialized_

It's never used.

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/797543002

Cr-Commit-Position: refs/heads/master@{#25770}

src/isolate.h

@@ -1232,7 +1232,6 @@ class Isolate {
   Counters* counters_;
   CodeRange* code_range_;
   base::RecursiveMutex break_access_;
-  base::Atomic32 debugger_initialized_;
   Logger* logger_;
   StackGuard stack_guard_;
   StatsTable* stats_table_;