Force full rebuilding after clang rolls.

BUG=chromium:464657
LOG=n

Review URL: https://codereview.chromium.org/987063002

Cr-Commit-Position: refs/heads/master@{#27081}

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.