tcmalloc: don't include emergency_malloc_for_stacktrace.cc

Based on the gperftools' Makefile, emergency_malloc_for_stacktrace.cc
is used only when ENABLE_EMERGENCY_MALLOC is defined. We don't define
this flag in Chromium, so we should build only with
fake_stacktrace_scope.cc, which provides dummy definitions of the same
symbols.

BUG=724399,b:70905156

Change-Id: I03404df00ef1d530bd04d35634f1c29fe8bb56db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1586100Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654665}

base/allocator/BUILD.gn

@@ -187,8 +187,8 @@ if (use_allocator == "tcmalloc") {
     if (use_new_tcmalloc) {
       sources -= [ "$tcmalloc_dir/src/base/cycleclock.h" ]
       sources += [
-        "$tcmalloc_dir/src/emergency_malloc_for_stacktrace.cc",
         "$tcmalloc_dir/src/fake_stacktrace_scope.cc",
+        "$tcmalloc_dir/src/maybe_emergency_malloc.h",
       ]
     }