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On 32-bit Windows builds where DCHECK is enabled, ComInitCheckHook
hot-patches CoCreateInstance() to DCHECK that it is called in
appropriate circumstances. The hot-patching method can only be used once
(details in base/win/com_init_check_hook.cc), and Chrome's usage may
conflict with other drivers that may want to hot-patch the same method
(example in https://crbug.com/737090).

This CL adds answers to the following questions that may arise when
investigating hot-patch failures.

1) Is hot-patching failing due to a bug where the HookManager singleton
   is instantiated twice? This can happen if base is linked in twice.
   This question is answered by printing the bytes that would be written
   to the hot-patch area, so they can be compared with the current
   values.
2) Is hot-patching failing due to a previous failure to revert the
   hot-patch? This question is answered by DCHECKing that reverting the
   hot-patch never fails. (The code previously just assumed that.)

Bug: 877868
Change-Id: I94f81b3816417447d31fc700c6b81d8325222d1a
Reviewed-on: https://chromium-review.googlesource.com/1189143
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586375}