[ios] Fix interaction of build/clobber.py & ios/build/tools/setup-gn.py

The script build/clobber.py tried to parse the build.ninja when
clobbering a directory and use it as a template, however if the
parsing failed, it generated a broken build.ninja file.

The script ios/build/tools/setup-gn.py would create multiple
empty build directory with just args.gn and a build.ninja file
that would result in invoking `gn gen` as a first step.

However, the build.ninja created by setup-gn.py could not be
parsed by clobber.py since it did not include minimum required
version of ninja command. Also, setup-gn.py did not recreate
the file if they existed, even if they were incorrect.

All this caused build/clobber.py to sometimes leave a build
directory in an unbuildable state requiring the developer to
run `gn gen` manually.

To fix this, change setup-gn.py to always overwrite build.ninja
file, to use a format that can be parsed by clobber.py and fix
clobber.py to write a build.ninja file that is valid if parsing
fails.

Always overwrite the build.ninja file to ensure that `--check`
is added to `gn gen` invocation (as locally running `gn gen`
without `--check` even once would disable the local check of the
deps and later be reported by Chrome iOS developers as a bug).

Bug: 1102331
Change-Id: Iabe906ac5f676f1b837c6ebb03ee32d73c7b6cf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283143
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786208}
GitOrigin-RevId: 75bb49ad3432cb331ae13058d2029ac060ab3543

clobber.py

@@ -89,13 +89,15 @@ def delete_build_dir(build_dir):
       f.write(build_commands)
     else:
       # Couldn't parse the build.ninja file, write a default thing.
-      f.write('''rule gn
-command = gn -q gen //out/%s/
-description = Regenerating ninja files
+      f.write('''ninja_required_version = 1.7.2
+
+rule gn
+  command = gn -q gen //out/%s/
+  description = Regenerating ninja files
 
 build build.ninja: gn
-generator = 1
-depfile = build.ninja.d
+  generator = 1
+  depfile = build.ninja.d
 ''' % (os.path.split(build_dir)[1]))
 
   # Write a .d file for the build which references a nonexistant file. This