Supress memory-leak warning on PersistentSampleMap.

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 # running overnight and removing outdated bugs/suppressions.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 354 matching lines @@
 }
 {
    Intentional leak for SampleMap (stores SparseHistogram counts).
    Memcheck:Leak
    ...
    fun:_ZN4base9SampleMap10AccumulateEii
    ...
    fun:_ZN4base15SparseHistogram*
 }
 {
+   Intentional leak for PersistentSampleMap (stores SparseHistogram counts).
+   Memcheck:Leak
+   ...
+   fun:_ZN4base19PersistentSampleMap10AccumulateEii
+   ...
+   fun:_ZN4base15SparseHistogram*
+}
+{
    Intentional leak for BucketRanges.
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN4base12_GLOBAL__N_120CreateRangesFromDataEPijm
    fun:_ZN4base28PersistentHistogramAllocator15CreateHistogramEPNS0_23PersistentHistogramDataE
 }
 {
    bug_73299 (Intentional)
    Memcheck:Leak
    fun:_Znw*
@@ skipping 3057 matching lines @@
    ...
    fun:_ZN7content28ShellURLRequestContextGetter20GetURLRequestContextEv
    fun:_ZN7content21ChromeAppCacheService20InitializeOnIOThreadERKN4base8FilePathEPNS_15ResourceContextEPN3net23URLRequestContextGetterE13scoped_refptrIN7storage20SpecialStoragePolicyEE
 }
 {
    bug_587544_b
    Memcheck:Leak
    ...
    fun:_ZN4base4BindIMN7content21ChromeAppCacheServiceEFvRKNS_8FilePathEPNS1_15ResourceContextEPN3net23URLRequestContextGetterE13scoped_refptrIN7storage20SpecialStoragePolicyEEEJPS2_S3_S7_SB_IS9_ESE_EEENS_8CallbackINS_8internal22MakeUnboundRunTypeImplIT_JDpT0_EE4TypeEEESM_DpOSN_
 }