Re-add SitePerProcessBrowserTest.TestChildProcessImportance to filter.

I deleted it incorrectly in https://crrev.com/c/1862137 but it's
failing.

TBR=altimin@chromium.org

Bug: 1004786
Change-Id: Ibfdc2f4f3d720b6f40e66cb02ed8f96ef4441380
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881005Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709752}

testing/buildbot/filters/bfcache.content_browsertests.filter

@@ -40,6 +40,9 @@
 -SitePerProcessBrowserTest.SubframePendingAndBackToSameSiteInstance
 -SitePerProcessBrowserTest.SwapOutACKArrivesPriorToProcessShutdownRequest
 
+# https://crbug.com/1004786 Android only
+-SitePerProcessBrowserTest.TestChildProcessImportance
+
 # Expect the swapped out RenderFrameHost to have a replacement proxy. This won't
 # happen when the BackForwardCache is used to store the old document.
 -RenderFrameHostManagerTest.RenderViewInitAfterNewProxyAndProcessKill