Fix valgrind warnings in YUV scaling code.

tools/valgrind/memcheck/suppressions.txt

 # There are four 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 # running overnight and removing outdated bugs/suppressions.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 3418 matching lines @@
    Memcheck:Unaddressable
    ...
    fun:_ZN5blink31WorkerFileSystemCallbacksBridge4stopEv
    fun:_ZN5blink31WorkerFileSystemContextObserver10notifyStopEv
    fun:_ZN7WebCore13WorkerContext21notifyObserversOfStopEv
    fun:_ZN7WebCore29WorkerThreadShutdownStartTask11performTaskEPNS_22ScriptExecutionContextE
    fun:_ZN7WebCore13WorkerRunLoop4Task11performTaskERKS0_PNS_22ScriptExecutionContextE
    fun:_ZN7WebCore13WorkerRunLoop15runCleanupTasksEPNS_13WorkerContextE
 }
 {
-   bug_113076
-   Memcheck:Uninitialized
-   fun:_ZN5mediaL19ConvertYUVToRGB32_CEhhhPh
-   fun:_ZN5media35LinearScaleYUVToRGB32RowWithRange_CEPKhS1_S1_Phiii
-   fun:_ZN5media23ScaleYUVToRGB32WithRectEPKhS1_S1_Phiiiiiiiiiii
-}
-{
    bug_115294
    Memcheck:Leak
    fun:_Znw*
    ...
    fun:_ZN17AudioRendererHost14OnCreateStreamEiRK15AudioParameters
 }
 {
    bug_115419_1
    Memcheck:Uninitialized
    fun:fetch_texel_2d_f_rgba8888
@@ skipping 2910 matching lines @@
    obj:/lib/x86_64-linux-gnu/libc-2.15.so
    fun:_IO_file_write@@GLIBC_2.2.5
    fun:new_do_write
    fun:_IO_do_write@@GLIBC_2.2.5
    fun:_IO_switch_to_get_mode
    fun:_IO_file_seekoff@@GLIBC_2.2.5
    fun:ftell
    fun:_ZN4base12TruncateFileEP8_IO_FILE
    fun:_ZN12_GLOBAL__N_132PrefixSetTest_SizeTRecovery_Test8TestBodyEv
 }