[Spellcheck] Add links to design doc in idle time spell checker classes

Tbr: yosin@chromium.org
NoTry: True
Change-Id: I52119806a656cf103d98ed09d328a354973145d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580619Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653443}

third_party/blink/renderer/core/editing/spellcheck/cold_mode_spell_check_requester.h

@@ -20,6 +20,7 @@ class SpellCheckRequester;
 // mode invocation. Not to be confused with SpellCheckRequester. The class
 // iteratively checks the editing host currently focused when the document is
 // idle.
+// See design doc for details: https://goo.gl/zONC3v
 class ColdModeSpellCheckRequester
     : public GarbageCollected<ColdModeSpellCheckRequester> {
  public:

third_party/blink/renderer/core/editing/spellcheck/hot_mode_spell_check_requester.h

@@ -15,6 +15,7 @@ class SpellCheckRequester;
 
 // This class is only supposed to be used by IdleSpellCheckController in hot
 // mode invocation. Not to be confused with SpellCheckRequester.
+// See design doc for details: https://goo.gl/zONC3v
 class HotModeSpellCheckRequester {
   STACK_ALLOCATED();
 

third_party/blink/renderer/core/editing/spellcheck/idle_spell_check_controller.h

@@ -26,6 +26,7 @@ class SpellCheckRequester;
   V(InColdModeInvocation)
 
 // Main class for the implementation of idle time spell checker.
+// See design doc for details: https://goo.gl/zONC3v
 class CORE_EXPORT IdleSpellCheckController final
     : public GarbageCollectedFinalized<IdleSpellCheckController>,
       public DocumentShutdownObserver {