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Crash reports beginning from Chrome V48 suggest that CreateUTIFromExtension may
return nil. This causes a crash when the uti string is added to the
NSMutableSet which holds the uti strings corresponding to the extension
dropdown. This CL does a nil check before adding the string to the set, which
should prevent the crash.

BUG=630101

Review-Url: https://codereview.chromium.org/2164223003
Cr-Commit-Position: refs/heads/master@{#407114}