gn: include CR_CLANG_REVISION in defines for clang

This matches gyp behaviour.

R=brettw@chromium.org,thakis@chromium.org
BUG=335824

Review URL: https://codereview.chromium.org/558853002

Cr-Original-Commit-Position: refs/heads/master@{#294098}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b1204ef04bc88d632d7458ae070af34b62eafe33

config/compiler/BUILD.gn

@@ -103,6 +103,18 @@ config("compiler") {
     }
   }
 
+  if (is_clang && !is_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 system rebuild things when their commandline changes). Nothing
+    # should ever read this define.
+    defines += [
+      "CR_CLANG_REVISION=" +
+          exec_script(
+              "//tools/clang/scripts/posix-print-revision.py", [], "value")
+    ]
+  }
+
   # Mac-specific compiler flags setup.
   # ----------------------------------
   if (is_mac || is_ios) {