Port enabling rtti for cfi.

Partially port https://codereview.chromium.org/1243373003/

BUG=chromium:515782
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1329553002

Cr-Commit-Position: refs/heads/master@{#30497}

build/standalone.gypi

@@ -1248,6 +1248,12 @@
               '-fno-sanitize-trap=cfi',
               '-fsanitize-recover=cfi',
             ],
+            'cflags_cc!': [
+              '-fno-rtti',
+            ],
+            'cflags!': [
+              '-fno-rtti',
+            ],
             'ldflags': [
               '-fno-sanitize-trap=cfi',
               '-fsanitize-recover=cfi',