Re-enable FullscreenVideoTest#testFullscreenDimensions

This test had been disabled due to flakiness. My change to frame eviction was
the likely cause. This was fixed by https://chromium-review.googlesource.com/c/chromium/src/+/1355845

I also separately ran an experiment which confirmed that the suspected mode of
failure occurs frequently during testing. Where currently visible surfaces are
being evicted. My change invalidated the current id, with the goal of allocating
new ids closer to usage. Unfortunately on Android the eviction process itself
can be the signal of needing a new id.

Now we simply allocate if visible.

TBR=peconn@chromium.org

Bug: 906840
Change-Id: I917a2e230ee472a48858b56feca2c96dc7bd7761
Reviewed-on: https://chromium-review.googlesource.com/c/1358652Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613091}

chrome/android/javatests/src/org/chromium/chrome/browser/video/FullscreenVideoTest.java

@@ -102,7 +102,6 @@ public class FullscreenVideoTest {
      */
     @Test
     @MediumTest
-    @FlakyTest(message = "crbug.com/906840")
     public void testFullscreenDimensions() throws InterruptedException, TimeoutException {
         String url =
                 mTestServerRule.getServer().getURL("/content/test/data/media/video-player.html");