WebView: Silence error for missing variations seed

VariationsUtils.readSeedFile shouldn't log errors for missing seed
files, which is expected on an app's first run.

BUG=869248

Change-Id: I92d88d10d6806d402b6b64c2f870e76860bf0d69
Reviewed-on: https://chromium-review.googlesource.com/1157116Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579525}

android_webview/java/src/org/chromium/android_webview/VariationsUtils.java

@@ -87,10 +87,11 @@ public class VariationsUtils {
         }
     }
 
-    // Silently returns null in case of incomplete/corrupt/missing seed, which is expected in case
+    // Silently returns null in case of a missing or truncated seed, which is expected in case
     // of an incomplete downoad or copy. Other IO problems are actual errors, and are logged.
     @Nullable
     public static SeedInfo readSeedFile(File inFile) {
+        if (!inFile.exists()) return null;
         FileInputStream in = null;
         try {
             in = new FileInputStream(inFile);