WebView: Fix CloseGuard error in VariationsSeedLoader

The WebView requesting a variations seed opens a file descriptor to
receive the seed. We should close it after the request.

BUG=857223

Change-Id: I594067ce0c33bed2137bc39e5c133ba22bca0c04
Reviewed-on: https://chromium-review.googlesource.com/1117707
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571354}

android_webview/java/src/org/chromium/android_webview/VariationsSeedLoader.java

@@ -291,6 +291,7 @@ public class VariationsSeedLoader {
                 Log.e(TAG, "Faild requesting seed", e);
             } finally {
                 ContextUtils.getApplicationContext().unbindService(this);
+                VariationsUtils.closeSafely(mNewSeedFd);
             }
         }