Add heapcheck suppressionfor BackgrounApplicationListModelTest.LoadExplicitExtensions

tools/heapcheck/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 # running overnight and removing outdated bugs/suppressions.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 1242 matching lines @@
 }
 {
    bug_73268b
    Heapcheck:Leak
    ...
    fun:bool ::InitCanonical
    fun:GURL
    ...
    fun:ExtensionSpecialStoragePolicyTest_*
 }
+{
+   bug_73358
+   Heapcheck:Leak
+   fun:TestingProfile::GetRequestContextForExtensions
+   fun:ExtensionDataDeleter
+   fun:ExtensionService::ClearExtensionData
+   fun:ExtensionService::UninstallExtension
+   fun:BackgroundApplicationListModelTest_LoadExplicitExtensions_Test::TestBody
+}