Turn off -Wl,-z,defs when building with CFI diagnostics.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index e5bc454a7cc9e18c8741e72d85b43da84a0bc3c2..c0bb69cbd4ab7c44145f5a395e90a74ec23adafe 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -3639,7 +3639,7 @@
       },
     }],
     # -Wl,-z,-defs doesn't work with the sanitiziers, http://crbug.com/452065
-    ['(OS=="linux" or OS=="android") and asan==0 and msan==0 and tsan==0 and ubsan==0 and ubsan_vptr==0', {
+    ['(OS=="linux" or OS=="android") and asan==0 and msan==0 and tsan==0 and ubsan==0 and ubsan_vptr==0 and cfi_diag==0', {

[Nico, 2015/08/10 19:13:09]

This looks good.

       'target_defaults': {
         'ldflags': [
           '-Wl,-z,defs',
@@ -6104,6 +6104,12 @@
               '-fno-sanitize-trap=cfi',
               '-fsanitize-recover=cfi',
             ],
+            'cflags_cc!': [
+              '-fno-rtti',
+            ],
+            'cflags!': [
+              '-fno-rtti',
+            ],

[Nico, 2015/08/10 19:13:09]

That's a pretty big behavioral difference. Is this really needed? How much
better do diagnostics get with this?

[pcc1, 2015/08/10 19:30:04]

This is needed to see the dynamic types of objects that fail the CFI check.
(Note that we already do something similar for ubsan==1 and ubsan_vptr==1, for
largely the same reason.)

[Nico, 2015/08/11 22:09:13]

But CFI is supposed to possibly be shipped to users while ubsan isn't, right? Or
is cfi_diag the not-shipping config? (In which case the names are similar enough
that it'd be easy to accidentally ship the wrong thing)

[pcc1, 2015/08/11 22:37:57]

cfi_diag==1 is the non-shipping config and is not the default. There are other
things we can do to make sure that we ship the right config, such as adding an
automated test that navigates the browser to chrome://badcastcrash which
intentionally causes a crash under the intended shipping config, cfi_vptr=1
cfi_diag=0.

             'ldflags': [
               '-fno-sanitize-trap=cfi',
               '-fsanitize-recover=cfi',