[MemSheriff] Green up DrMemory bots.

Widen a suppression and exclude two tests.

BUG=470517, 470507, 463261
TBR=eugenis@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1001843003

Cr-Commit-Position: refs/heads/master@{#322145}

tools/valgrind/drmemory/suppressions.txt

@@ -755,7 +755,7 @@ UNADDRESSABLE ACCESS
 name=http://crbug.com/463261
 ...
 blink_web.dll!blink::WebFrameWidgetImpl::selectionBounds
-content.dll!content::RenderWidget::GetSelectionBounds
+...
 content.dll!content::RenderWidget::UpdateSelectionBounds
 content.dll!content::RenderWidget::willBeginCompositorFrame
 cc.dll!base::internal::InvokeHelper<>::MakeItSo

tools/valgrind/gtest_exclude/content_browsertests.gtest-drmemory.txt

@@ -35,3 +35,6 @@ WebRtcBrowserTest.CallAndModifyStream
 # http://crbug.com/464033
 RenderFrameHostImplBrowserTest.IsFocused_AtLoad
 RenderFrameHostImplBrowserTest.IsFocused_Widget
+
+# http://crbug.com/470507
+SRC_ClearKey/EncryptedMediaTest.Playback_VideoClearAudio_WebM/0

tools/valgrind/gtest_exclude/media_unittests.gtest-drmemory.txt

+# http://crbug.com/470517
+VideoRendererImplTest.Underflow