Add a assert for running a sanitizer in debug.

I found that msan doesn't link in debug builds. It fails linking with an
obscure error. To prevent other people from having this problem, this patch
adds an assert for this case.

Review URL: https://codereview.chromium.org/1326453003

Cr-Original-Commit-Position: refs/heads/master@{#346764}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1131f71c0604dfbe6e1f2d3ef6dcb8d288c7e4f6

config/sanitizers/sanitizers.gni

@@ -23,3 +23,17 @@ declare_args() {
   # https://code.google.com/p/sawbuck/wiki/SyzyASanHowTo
   is_syzyasan = false
 }
+
+# MSan only links Chrome properly in release builds (brettw -- 9/1/2015). The
+# same is possibly true for the other sanitizers. But regardless of whether it
+# links, one would normally never run a sanitizer in debug mode. Running in
+# debug mode probably indicates you forgot to set the "is_debug = false" flag
+# in the build args.
+#
+# If you find a use-case where you want to compile a sanitizer in debug mode
+# and have verified it works, you can remove it from this condition. But please
+# balance how common your use-case is versus how easy it is to forget to set
+# the release flag, have an unpleasant time trying to use the sanitizer, and
+# have to rebuild. For one-off builds, just comment this out locally.
+assert(!is_debug || !(is_msan || is_asan || is_lsan || is_tsan),
+       "Sanitizers should generally be used in release (set is_debug=false).")