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Raises an NSInvalidArgumentException if call setObject for NSMutableDictionary
with a nil value. Dictionary uses NSNull instead of nil.
Selected text marker range may be nil sometimes
(see BrowserAccessibilityCocoa::selectedTextMarkerRange), that leads to a crashe
in BrowserAccessibilityManagerMac::GetUserInfoForSelectedTextChangedNotification().

R=aboxhall@chromium.org, dmazzoni@chromium.org, dtseng@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2161623002
Cr-Commit-Position: refs/heads/master@{#407194}