Implement WebViewChromium.freeMemory.

This API now an intentional no-op as Chromium will manage
WebView memory automatically.

Bug: 5012874
Change-Id: I0d72a80333704bb88af96e9492a9029b02c93280
(cherry picked from commit d43c55dd91cf7dd1bcd18d01e16acd5e55de0471)
diff --git a/chromium/java/com/android/webview/chromium/WebViewChromium.java b/chromium/java/com/android/webview/chromium/WebViewChromium.java
index d75b457..d84b31e 100644
--- a/chromium/java/com/android/webview/chromium/WebViewChromium.java
+++ b/chromium/java/com/android/webview/chromium/WebViewChromium.java
@@ -588,7 +588,7 @@
     @Override
     public void freeMemory() {
         checkThread();
-        UnimplementedWebViewApi.invoke();
+        // Intentional no-op. Memory is managed automatically by Chromium.
     }
 
     @Override