Fix `gn clean`.

The way `gn clean` preserves enough of the toplevel build.ninja
file in order to be able to recreate itself is kind of hacky; it
looks for a certain number of blank lines and then discards the
rest. When we added the `ninja_required_version` field to the
file, that added a blank line, but we didn't bump the number of
blank lines to look for by one. This meant that we discarded
the rule that would actually regenerate ninja.

As a result, `gn clean` would break ninja's ability to regenerate
the file. This CL fixes that.

R: brettw@chromium.org
Bug: 757035
Change-Id: Ie200c5705d93a9327741f52b194a35c31b66ecce
Reviewed-on: https://chromium-review.googlesource.com/621589
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#495682}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7a152fc8b6727dfbf1616043f314d99079605e45
diff --git a/tools/gn/command_clean.cc b/tools/gn/command_clean.cc
index 8903986..70def6d 100644
--- a/tools/gn/command_clean.cc
+++ b/tools/gn/command_clean.cc
@@ -35,7 +35,7 @@
     result.push_back('\n');
     if (line.empty())
       ++num_blank_lines;
-    if (num_blank_lines == 2)
+    if (num_blank_lines == 3)
       break;
   }