Initialize user_gesture_observed_ in NavigationController constructor.

tools/valgrind/memcheck/suppressions.txt

 # There are three kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 881 matching lines @@
    fun:_ZN3IPC12ChannelProxy7Context15OnChannelOpenedEv
 }
 {
    # Also bug_17795
    bug_17996
    Memcheck:Leak
    ...
    fun:_ZN7WebCore8SVGNames4initEv
 }
 {
-   bug_18268
-   Memcheck:Cond
-   fun:_ZN20NavigationController22IsLikelyAutoNavigationEN4base9TimeTicksE
-   fun:_ZN20NavigationController28RendererDidNavigateToNewPageERK32ViewHostMsg_FrameNavigate_ParamsPb
-}
-{
    bug_18654
    Memcheck:Leak
    ...
    fun:_ZN12WebFrameImplD0Ev
    fun:_ZN20WebFrameLoaderClient20frameLoaderDestroyedEv
    fun:_ZN7WebCore11FrameLoaderD1Ev
    fun:_ZN7WebCore5FrameD1Ev
    fun:_ZN7WebCore17InspectorResourceD1Ev
    fun:_ZN3WTF10RefCountedIN7WebCore17InspectorResourceEE5derefEv
    fun:_ZN7WebCore19InspectorControllerD1Ev
@@ skipping 335 matching lines @@
    fun:_ZN10JSONWriter15BuildJSONStringEPK5Valueib
    fun:_ZN10JSONWriter15BuildJSONStringEPK5Valueib
    fun:_ZN10JSONWriter15BuildJSONStringEPK5Valueib
    fun:_ZN10JSONWriter23WriteWithOptionalEscapeEPK5ValuebbPSs
    fun:_ZN10JSONWriter5WriteEPK5ValuebPSs
    fun:_ZN25JSONStringValueSerializer9SerializeERK5Value
    fun:_ZN11PrefService13SerializeDataEPSs
    fun:_ZN11PrefService19SavePersistentPrefsEv
    fun:_ZN16browser_shutdown8ShutdownEv
 }