Fix Valgrind UMR errors by initializing thumbnail pixels.

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 # running overnight and removing outdated bugs/suppressions.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 591 matching lines @@
    fun:_ZN7WebCore10CSSRuleSet17addRulesFromSheetEPNS_13CSSStyleSheetERKNS_19MediaQueryEvaluatorEPNS_16CSSStyleSelectorE
    ...
    fun:_ZN7WebCore16CSSStyleSelectorC1EPNS_8DocumentERKNS_6StringEPNS_14StyleSheetListEPNS_13CSSStyleSheetEbb
    fun:_ZN7WebCore8Document6attachEv
    fun:_ZN7WebCore5Frame11setDocumentEN3WTF10PassRefPtrINS_8DocumentEEE
    fun:_ZN7WebCore11FrameLoader5beginERKNS_4KURLEbPNS_14SecurityOriginE
    fun:_ZN7WebCore11FrameLoader4initEv
    fun:_ZN7WebCore5Frame4initEv
 }
 {
-   bug_17067
-   Memcheck:Value4
-   fun:rgb_ycc_convert
-   fun:pre_process_data
-   fun:process_data_simple_main
-   fun:chromium_jpeg_write_scanlines
-   fun:_ZN3gfx9JPEGCodec6EncodeEPKhNS0_11ColorFormatEiiiiPSt6vectorIhSaIhEE
-   fun:_ZN7history8TopSites16SetPageThumbnailERK4GURLRK8SkBitmapRK14ThumbnailScore
-   fun:_ZN7history34TopSitesTest_SetPageThumbnail_Test8TestBodyEv
-}
-{
    bug_17113
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN18ResourceDispatcher12CreateBridgeERKSsRK4GURLS4_S4_S1_S1_S1_iiN12ResourceType4TypeEjii
    fun:_ZN11webkit_glue20ResourceLoaderBridge6CreateERKSsRK4GURLS5_S5_S2_S2_S2_iiN12ResourceType4TypeEii
    fun:_ZN11webkit_glue16WebURLLoaderImpl7Context5StartERKN6WebKit13WebURLRequestEPNS_20ResourceLoaderBridge16SyncLoadResponseE
    fun:_ZN11webkit_glue16WebURLLoaderImpl18loadAsynchronouslyERKN6WebKit13WebURLRequestEPNS1_18WebURLLoaderClientE
 }
 {
    bug_17291
@@ skipping 555 matching lines @@
    fun:_ZN3WTF31dispatchFunctionsFromMainThreadEv
    fun:_Z18DispatchToFunctionIPFvvEEvT_RK6Tuple0
    fun:_ZN16RunnableFunctionIPFvvE6Tuple0E3RunEv
    fun:_ZN11MessageLoop7RunTaskEP4Task
    fun:_ZN11MessageLoop21DeferOrRunPendingTaskERKNS_11PendingTaskE
    fun:_ZN11MessageLoop6DoWorkEv
    fun:_ZN4base18MessagePumpDefault3RunEPNS_11MessagePump8DelegateE
    fun:_ZN11MessageLoop11RunInternalEv
    fun:_ZN11MessageLoop10RunHandlerEv
 }