chromeos: reenables pdf tests for mus_browser_tests

These tests now pass since PDF were made to work (755440).

BUG=755272
TEST=test only change
TBR=msw@chromium.org

Change-Id: Id79e8aa6dbf810a5f6789943e5b6a2665e2dc3e4
Reviewed-on: https://chromium-review.googlesource.com/761956Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515472}

testing/buildbot/filters/mojo.fyi.mus.browser_tests.filter

@@ -9,11 +9,6 @@
 -TabCaptureApiPixelTest.OffscreenTabEndToEnd
 -TabCaptureApiPixelTest.OffscreenTabEvilTests
 
-# TODO: fix, see http://crbug.com/755272.
--PDFExtensionTest.RedirectsFailInPlugin
--SaveType/SavePageOriginalVsSavedComparisonTest.ObjectElementsViaHttp/0
--SaveType/SavePageOriginalVsSavedComparisonTest.ObjectElementsViaFile/0
-
 # TODO: fix, see http://crbug.com/755303.
 -StartupMetricsTest.ReportsValues