Add heapcheck suppression for PrintPreviewTabControllerTest.MultiplePreviewTabs.

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 # running overnight and removing outdated bugs/suppressions.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 1037 matching lines @@
    Heapcheck:Leak
    fun:base::BaseTimer::Start
    fun:ReloadButtonGtk::OnClicked
    fun:ReloadButtonGtk::OnClickedThunk
    fun:g_closure_invoke
    fun:ReloadButtonGtkTest_*_Test::TestBody
    fun:testing::HandleExceptionsInMethodIfSupported
    fun:testing::Test::Run
 }
 {
-   bug_63386
+   bug_63386_a
    Heapcheck:Leak
    ...
    fun:PrintPreviewTabControllerTest_GetOrCreatePreviewTab_Test::TestBody
 }
+{
+   bug_63386_b
+   Heapcheck:Leak
+   ...
+   fun:PrintPreviewTabControllerTest_MultiplePreviewTabs_Test::TestBody
+}