MSVC Compatibility: Change DropIndex::value to an int

DropIndex::value is treated as an int, not a size_t.

BUG=

Change-Id: I5933c10a8b44758606e7a8bfd124e4853f42fd5c
Reviewed-on: https://chromium-review.googlesource.com/1049009
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556709}

chrome/browser/ui/views/frame/browser_root_view.h

@@ -23,7 +23,7 @@ class BrowserRootView : public views::internal::RootView {
   struct DropIndex {
     // The index within the tabstrip to drop on/before (see
     // |insert_before_index| below).
-    size_t value = 0;
+    int value = 0;
 
     // If true, the dropped item should be inserted before |tab_index|.
     // If false, the dropped item should replace the tab at |tab_index|.