Suppress an uninitialized read in ThreadHeap<>::sweepNormalPages

BUG=407396
NOTRY=true
TBR=ager@chromium.org
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/502233002

Cr-Commit-Position: refs/heads/master@{#291809}

tools/valgrind/drmemory/suppressions_full.txt

@@ -2236,3 +2236,9 @@ content.dll!content::WebContentsImpl::CreateRenderViewForRenderManager
 content.dll!content::NavigationControllerImpl::LoadEntry
 ...
 *!extensions::ExtensionHost::LoadInitialURL
+
+UNINITIALIZED READ
+name=bug_407396
+blink_platform.dll!blink::ThreadHeap<>::sweepNormalPages
+blink_platform.dll!blink::ThreadHeap<>::sweep
+blink_platform.dll!blink::ThreadState::performPendingSweep