[fuchsia] Re-enable disabled content_unittests

Tried multiple runs locally with no flakes, attempting to re-enable.

Bug: 760687
Change-Id: I71a077e52028182dde777c79aea337dbcf1f6d9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592627
Auto-Submit: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656486}

testing/buildbot/filters/fuchsia.content_unittests.filter

@@ -8,10 +8,6 @@
 -RendererAudioOutputStreamFactoryIntegrationTest.StreamIntegrationTest
 -WebContentsAudioInputStreamTest.MirroringNothingWithTargetChange/0
 
-# Flaky: https://crbug.com/760687.
--CacheStorageManagerTest.GetAllOriginsUsageWithOldIndex
--CacheStorageManagerTest.GetOriginSizeWithOldIndex
-
 # Flaky: https://crbug.com/759653.
 -RenderWidgetHostViewAuraTest.Resize