Include -fsanitize=float-cast-overflow in is_ubsan=true builds.

Also add a comment about the current state of ubsan.

BUG=669642

Review-Url: https://codereview.chromium.org/2550593004
Cr-Original-Commit-Position: refs/heads/master@{#436093}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2e147959b92ac70c0eb3e0568ae664d39614b68c

config/sanitizers/BUILD.gn

@@ -428,6 +428,9 @@ config("tsan_flags") {
   }
 }
 
+# UBSan is still in bring-up.  Chrome builds with is_ubsan=true, but if you
+# set is_ubsan=true is_ubsan_no_recover = true, then many tests are still
+# failing.
 config("ubsan_flags") {
   cflags = []
   if (is_ubsan) {
@@ -438,6 +441,7 @@ config("ubsan_flags") {
       # enabled. See http://crbug.com/489901
       # "-fsanitize=bounds",
       "-fsanitize=float-divide-by-zero",
+      "-fsanitize=float-cast-overflow",
       "-fsanitize=integer-divide-by-zero",
       "-fsanitize=null",
       "-fsanitize=object-size",