FormatUtil: Fix Math#round() truncation error flagged by error-prone

Building with Bazel@HEAD: [1] is failing with this error message:

  FormatUtil.java:129: error: [MathRoundIntLong] Math.round(Integer) results in truncation
  | int p = Math.abs(Math.round(delta * 100 / size));
  | ^
  | (see https://errorprone.info/bugpattern/MathRoundIntLong)
  | Did you mean 'int p = Math.abs(Ints.saturatedCast(delta * 100 / size));'?

To rectify, replace Math.round() with Ints.saturatedCast() and borrow
this method from Guava library because Guava cannot be currently used in
GWT client code.

[1] https://github.com/bazelbuild/bazel/issues/5944
Change-Id: I1c88102d4d027796594fb7cdcde4f0ec4921d4ad

gerrit-gwtui/src/main/java/com/google/gerrit/client/FormatUtil.java

@@ -137,7 +137,25 @@ public class FormatUtil {
     if (size == 0) {
       return Resources.C.notAvailable();
     }
-    int p = Math.abs(Math.round(delta * 100 / size));
+    int p = Math.abs(saturatedCast(delta * 100 / size));
     return p + "%";
   }
+
+  /**
+   * Returns the {@code int} nearest in value to {@code value}.
+   *
+   * @param value any {@code long} value
+   * @return the same value cast to {@code int} if it is in the range of the {@code int} type,
+   *     {@link Integer#MAX_VALUE} if it is too large, or {@link Integer#MIN_VALUE} if it is too
+   *     small
+   */
+  private static int saturatedCast(long value) {
+    if (value > Integer.MAX_VALUE) {
+      return Integer.MAX_VALUE;
+    }
+    if (value < Integer.MIN_VALUE) {
+      return Integer.MIN_VALUE;
+    }
+    return (int) value;
+  }
 }