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This CL relands r555005 which was reverted since it introduced flakiness. The
flakiness was most probably since we ended up creating the
ExtensionTestMessageListener too late for the
DeclarativeNetRequestBrowserTest_Packed.PageWhitelistingAPI test. The listener
was created in SetUpOnMainThread override while the InstalledLoader ends up
loading all extensions before this (during
BrowserMainLoop::CreateStartupTasks()). Hence a subsequent
ExtensionTestMessageListener::WaitUntilSatisfied() call would end up running
indefinitely. To get around this, only wait for the background page to load if
the background page isn't already ready.

BUG= 696822, 838536

Change-Id: I2b288c6e6703e5be588d480403b362bf7c30db4e
Reviewed-on: https://chromium-review.googlesource.com/1043271
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556319}