Add missing BASE_EXPORT to base::kDCheckIsFatal constant.

Bug: 832380
Change-Id: I0f059b801826147a8e86a7ba141215e95f57c734
Reviewed-on: https://chromium-review.googlesource.com/1011646Reviewed-by: David Grogan <dgrogan@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550676}

base/feature_list.h

@@ -47,7 +47,7 @@ struct BASE_EXPORT Feature {
 // DCHECKs have been built-in, and are configurable at run-time to be fatal, or
 // not, via a DcheckIsFatal feature. We define the Feature here since it is
 // checked in FeatureList::SetInstance(). See https://crbug.com/596231.
-extern const Feature kDCheckIsFatalFeature;
+extern BASE_EXPORT const Feature kDCheckIsFatalFeature;
 #endif  // DCHECK_IS_CONFIGURABLE
 
 // The FeatureList class is used to determine whether a given feature is on or