net: Implement canceling of all async operations in FileStream.

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 # running overnight and removing outdated bugs/suppressions.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 6016 matching lines @@
    bug_149748b
    Memcheck:Unaddressable
    ...
    fun:*SkPath*
    ...
    fun:_ZN8SkDevice8drawRectERK6SkDrawRK6SkRectRK7SkPaint
    fun:_ZN8SkCanvas8drawRectERK6SkRectRK7SkPaint
    fun:_ZN7WebCore15GraphicsContext10strokeRectERKNS_9FloatRectEf
    fun:_ZNK7WebCore13RenderSVGRect11strokeShapeEPNS_15GraphicsContextE
 }
+{
+   FileStream::Context can leak through WorkerPool by design
+   Memcheck:Leak
+   fun:_Znw*
+   fun:_ZN3net10FileStreamC1EPNS_6NetLogE
+   ...
+}
 
 #-----------------------------------------------------------------------
 # 4. These only occur on our Google workstations
 
 {
    bug_todo_getdelim
    Memcheck:Leak
    fun:malloc
    fun:getdelim
    ...
@@ skipping 32 matching lines @@
    Memcheck:Leak
    fun:malloc
    fun:get_peer_sock_name
    fun:_xcb_get_auth_info
    fun:xcb_connect_to_display_with_auth_info
    fun:_XConnectXCB
    fun:XOpenDisplay
    fun:_ZN4base18MessagePumpAuraX1118GetDefaultXDisplayEv
 }