Introduce cfi_vptr build flag.

Introduce cfi_vptr build flag.

This flag enables Clang's new control flow integrity mechanism for virtual table
calls, which is documented at:

http://clang.llvm.org/docs/ControlFlowIntegrity.html

BUG=464797
R=thakis@chromium.org

Committed: https://crrev.com/980ad01235a474d721c4e093488b88ec986295cf
Cr-Commit-Position: refs/heads/master@{#319534}

[Nico, 2015-03-06 21:13:26]

lgtm

https://codereview.chromium.org/985713005/diff/1/build/common.gypi
File build/common.gypi (right):

https://codereview.chromium.org/985713005/diff/1/build/common.gypi#newcode3003
build/common.gypi:3003: ['(use_lto==1 or cfi_vptr==1) and (target_arch=="ia32"
or target_arch=="x64")', {
alternatively use_lto could be set to 1 automatically if cfi_vptr is set (like
asan=1 used to set clang=1)

[pcc1, 2015-03-06 21:39:24]

The CQ bit was checked by pcc@chromium.org

[pcc1, 2015-03-06 21:39:24]

The patchset sent to the CQ was uploaded after l-g-t-m from thakis@chromium.org
Link to the patchset: https://codereview.chromium.org/985713005/#ps20001 (title:
"Set use_lto=1 automatically if cfi_vptr is set")

[commit-bot: I haz the power, 2015-03-06 21:39:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/985713005/20001

[commit-bot: I haz the power, 2015-03-07 00:16:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-07 00:17:28]

Patchset 2 (id:??) landed as
https://crrev.com/980ad01235a474d721c4e093488b88ec986295cf
Cr-Commit-Position: refs/heads/master@{#319534}