aw: Fix reachabilityFence version check

The method was added in P, not N.

Bug: 1084615
Change-Id: Ia0772c68b8190f3b0bc775e60c4aabefb59afa48
Tbr: torne@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208366Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770207}

android_webview/javatests/src/org/chromium/android_webview/test/AwContentsGarbageCollectionTest.java

@@ -301,7 +301,7 @@ public class AwContentsGarbageCollectionTest {
 
             gcAndCheckAllAwContentsDestroyed();
         } finally {
-            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
                 Reference.reachabilityFence(heldObject);
             }
         }