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

build/config/sanitizers/BUILD.gn

Index: build/config/sanitizers/BUILD.gn
diff --git a/build/config/sanitizers/BUILD.gn b/build/config/sanitizers/BUILD.gn
index 36bceec225a2954814f4020c313a0dd0ae25b6fa..36d45c1d1b6f1edf77020d965aec0f298c3fa702 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/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",