Remove duplicated file listed in BUILD.gn

The v8_base project can't be loaded in Visual Studio 2017 because
"src/zone/zone-allocator.h" is listed twice.

R=jgruber@chromium.org, yangguo@chromium.org

Change-Id: Ide2f27a7f8ffd3c57deb7f9ae16c55122d999d06
Reviewed-on: https://chromium-review.googlesource.com/998346Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52408}

AUTHORS

@@ -145,6 +145,7 @@ Vlad Burlik <vladbph@gmail.com>
 Vladimir Krivosheev <develar@gmail.com>
 Vladimir Shutoff <vovan@shutoff.ru>
 Wiktor Garbacz <wiktor.garbacz@gmail.com>
+Xiaoyin Liu <xiaoyin.l@outlook.com>
 Yong Wang <ccyongwang@tencent.com>
 Yu Yin <xwafish@gmail.com>
 Zac Hansen <xaxxon@gmail.com>

BUILD.gn

@@ -2294,7 +2294,6 @@ v8_source_set("v8_base") {
     "src/zone/accounting-allocator.cc",
     "src/zone/accounting-allocator.h",
     "src/zone/zone-allocator.h",
-    "src/zone/zone-allocator.h",
     "src/zone/zone-chunk-list.h",
     "src/zone/zone-containers.h",
     "src/zone/zone-handle-set.h",