Add support for a flag to explicitly disable ComInitCheckHook.

This is to be used only on bots where there's a conflict with necessary
(for purposes of testing) 3rd party drivers/components that employ
patching as part of their function. Currently WebRTC's "Win Builder"
will need to set this flag.

BUG=737090

Change-Id: If483b06aa9f0f8a797e41f429cc363979a5c01f3
Reviewed-on: https://chromium-review.googlesource.com/753591
Commit-Queue: Tommi <tommi@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517805}

base/win/com_init_check_hook.h

@@ -19,7 +19,8 @@ namespace win {
 
 #if DCHECK_IS_ON() && defined(ARCH_CPU_X86_FAMILY) &&             \
     defined(ARCH_CPU_32_BITS) && !defined(GOOGLE_CHROME_BUILD) && \
-    !defined(OFFICIAL_BUILD)
+    !defined(OFFICIAL_BUILD) &&                                   \
+    !defined(COM_INIT_CHECK_HOOK_DISABLED)  // See crbug/737090 for details.
 #define COM_INIT_CHECK_HOOK_ENABLED
 #endif