Fix styleguide violations from https://codereview.chromium.org/1681363003

* Don't use default capture (luckily not needed here)
* Don't use std::function -- since nothing's captured, the lambda can decay
  to a function pointer here

No behavior change.

BUG=none

Review URL: https://codereview.chromium.org/1705123004

Cr-Original-Commit-Position: refs/heads/master@{#375990}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 08feeb7e4150acb4dd1af413555023660682c3c8

tools/gn/command_help.cc

diff --git a/tools/gn/command_help.cc b/tools/gn/command_help.cc
index 18cb6d1..a486a72 100644
--- a/tools/gn/command_help.cc
+++ b/tools/gn/command_help.cc
@@ -222,17 +222,17 @@
     all_help_topics.push_back(entry.first);
 
   // Random other topics.
-  std::map<std::string, std::function<void()>> random_topics;
+  std::map<std::string, void(*)()> random_topics;
   random_topics["all"] = PrintAllHelp;
-  random_topics["buildargs"] = [=]() { PrintLongHelp(kBuildArgs_Help); };
-  random_topics["dotfile"] = [=]() { PrintLongHelp(kDotfile_Help); };
-  random_topics["grammar"] = [=]() { PrintLongHelp(kGrammar_Help); };
-  random_topics["input_conversion"] = [=]() {
+  random_topics["buildargs"] = []() { PrintLongHelp(kBuildArgs_Help); };
+  random_topics["dotfile"] = []() { PrintLongHelp(kDotfile_Help); };
+  random_topics["grammar"] = []() { PrintLongHelp(kGrammar_Help); };
+  random_topics["input_conversion"] = []() {
     PrintLongHelp(kInputConversion_Help);
   };
-  random_topics["label_pattern"] = [=]() { PrintLongHelp(kLabelPattern_Help); };
-  random_topics["runtime_deps"] = [=]() { PrintLongHelp(kRuntimeDeps_Help); };
-  random_topics["source_expansion"] = [=]() {
+  random_topics["label_pattern"] = []() { PrintLongHelp(kLabelPattern_Help); };
+  random_topics["runtime_deps"] = []() { PrintLongHelp(kRuntimeDeps_Help); };
+  random_topics["source_expansion"] = []() {
     PrintLongHelp(kSourceExpansion_Help);
   };
   random_topics["switches"] = PrintSwitchHelp;