Force full rebuilding after clang rolls.

build/standalone.gypi

Index: build/standalone.gypi
diff --git a/build/standalone.gypi b/build/standalone.gypi
index 3f7e3c630d11a2e84977b62038bd16f12556fb85..1aedbcd595904e3c5a5cef1f1e5cc28a4ea23197 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -558,6 +558,14 @@
         ['CC', '../<(clang_dir)/bin/clang-cl'],
       ],
     }],
+    ['(clang==1 or host_clang==1) and OS!="win"', {
+      # This is here so that all files get recompiled after a clang roll and
+      # when turning clang on or off.
+      # (defines are passed via the command line, and build systems rebuild
+      # things when their commandline changes). Nothing should ever read this
+      # define.
+      'defines': ['CR_CLANG_REVISION=<!(<(DEPTH)/tools/clang/scripts/update.sh --print-revision)'],
+    }],
     # TODO(yyanagisawa): supports GENERATOR==make
     #  make generator doesn't support CC_wrapper without CC
     #  in make_global_settings yet.