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Control Flow Integrity [1] indirect call (cfi-icall) checking can not
verify that dynamically resolved function pointers call their intended
function. Instead we place the function pointer ProtectedMemory, a
wrapper for keeping variables in read-only memory except for when they
are initialized.  After setting the pointer in protected memory we can
use the UnsanitizedCfiCall wrapper to disable cfi-icall checking when
calling it since we know it can not be tampered with.

[1] https://www.chromium.org/developers/testing/control-flow-integrity

BUG=1014902
R=thestig

Change-Id: Id55510f4d924bf8d4dd73332b7a8b18a9e6bc04c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864377
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706822}