Add quotes to CR_CLANG_REVISION define

clang_revision variable may has format, that represents arithmetical
expression. For example – 284979-2.
Althought it is stated that this define should not be used in code,
it is unsafe. It can be made as string, so it will perfectly do it's
job and be much more safe.

R=brettw@chromium.org, dpranke@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2674983002
Cr-Original-Commit-Position: refs/heads/master@{#448520}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3331c473d65bc3331cf14e9797b102fdbbf013ea

config/compiler/BUILD.gn

@@ -255,7 +255,7 @@ config("compiler") {
     # 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=$clang_revision" ]
+    defines += [ "CR_CLANG_REVISION=\"$clang_revision\"" ]
   }
 
   # Non-Mac Posix compiler flags setup.