AW: fix NarrowingCompoundAssignment warning

No change in behavior.

This changes two casts from implicit to explicit to resolve a
NarrowingCompoundAssignment errorprone warning.

Test: ninja system_webview_apk (warnings disappear)
Change-Id: I32d98a705e3bb3054668f25dfeb50e2af562c902
Reviewed-on: https://chromium-review.googlesource.com/777729Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517626}

android_webview/java/src/org/chromium/android_webview/AwLayoutSizer.java

@@ -96,8 +96,8 @@ public class AwLayoutSizer {
     public void onContentSizeChanged(int width, int height) {
         CommandLine commandLine = CommandLine.getInstance();
         if (!commandLine.hasSwitch(ContentSwitches.ENABLE_USE_ZOOM_FOR_DSF)) {
-            width *= mDIPScale;
-            height *= mDIPScale;
+            width = (int) (width * mDIPScale);
+            height = (int) (height * mDIPScale);
         }
         doUpdate(width, height, mPageScaleFactor);
     }