Add an externs.js file for D3.

Bug: 891310
Change-Id: Ie467f737e76cc0125b03723e16c3c12bdaf41187
Reviewed-on: https://chromium-review.googlesource.com/c/1271380Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601627}

third_party/d3/README.chromium

@@ -13,4 +13,4 @@ Description:
 A JavaScript library for manipulating documents based on data.
 
 Local Modifications:
-None
+Added externs.js to allow the closure compiler to compile code using d3.