suppress uninit error at extensions::UserScript::Pickle

TBR=markdittmer@chromium.org, kenrb@chromium.org, rdevlin.cronin@chromium.org
BUG=409865, 377978
NOTRY=true

Review URL: https://codereview.chromium.org/526373003

Cr-Commit-Position: refs/heads/master@{#292947}

tools/valgrind/drmemory/suppressions_full.txt

@@ -2197,3 +2197,11 @@ content.dll!content::WebContentsImpl::CreateRenderViewForRenderManager
 content.dll!content::NavigationControllerImpl::LoadEntry
 ...
 *!extensions::ExtensionHost::LoadInitialURL
+
+UNINITIALIZED READ
+name=bug_409865
+*!Pickle::WriteBool
+*!extensions::UserScript::Pickle
+*!extensions::`anonymous namespace'::Serialize
+*!extensions::`anonymous namespace'::LoadScriptsOnFileThread
+*!base::internal::RunnableAdapter<>::Run