[Interpreter] Do not write Ignition dispatch counters to file by default.

Since Ignition dispatch counters have been made accessible from
JavaScript via getIgnitionDispatchCounters() in [1], writing
them to a file at the end of the execution does not seem the best
default anymore.

Following this commit, a file is written only if d8 is invoked
with --trace-ignition-dispatches-output-file.

[1] https://crrev.com/905becd13b8696e126255decf130fdb9e1d9aa30

LOG=N
BUG=v8:4899

Review-Url: https://codereview.chromium.org/1943923002
Cr-Commit-Position: refs/heads/master@{#36015}

src/d8.cc

@@ -2495,7 +2495,8 @@ int Shell::Main(int argc, char* argv[]) {
     }
 
 #ifndef V8_SHARED
-    if (i::FLAG_ignition && i::FLAG_trace_ignition_dispatches) {
+    if (i::FLAG_ignition && i::FLAG_trace_ignition_dispatches &&
+        i::FLAG_trace_ignition_dispatches_output_file != nullptr) {
       WriteIgnitionDispatchCountersFile(isolate);
     }
 #endif

src/flag-definitions.h

@@ -299,10 +299,9 @@ DEFINE_BOOL(trace_ignition_codegen, false,
 DEFINE_BOOL(trace_ignition_dispatches, false,
             "traces the dispatches to bytecode handlers by the ignition "
             "interpreter")
-DEFINE_STRING(trace_ignition_dispatches_output_file,
-              "v8.ignition_dispatches_table.json",
+DEFINE_STRING(trace_ignition_dispatches_output_file, nullptr,
               "the file to which the bytecode handler dispatch table is "
-              "written")
+              "written (by default, the table is not written to a file)")
 
 // Flags for Crankshaft.
 DEFINE_BOOL(crankshaft, true, "use crankshaft")