Fix an issue in `gn analyze` when building all.

tools/mb/mb.py

Index: tools/mb/mb.py
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 76c63f5adc22793fc288b4b9db10a88c33d2e4b8..b9b11963c05aaa834c10596f099aac51ec3b0531 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -1284,8 +1284,11 @@ class MetaBuildWrapper(object):
       if 'invalid_targets' in gn_outp:
         outp['invalid_targets'] = gn_outp['invalid_targets']
       if 'compile_targets' in gn_outp:
-        outp['compile_targets'] = [
-          label.replace('//', '') for label in gn_outp['compile_targets']]
+        if 'all' in gn_outp['compile_targets']:
+          outp['compile_targets'] = ['all']
+        else:
+          outp['compile_targets'] = [
+              label.replace('//', '') for label in gn_outp['compile_targets']]
       if 'test_targets' in gn_outp:
         outp['test_targets'] = [
           labels_to_targets[label] for label in gn_outp['test_targets']]