blink: Delete misleading sentence in the Blink GC docs

The docs previously claimed that you don't need to add a Trace() method
to an on-heap class if it does not contain on-heap members. However,
this is no longer true.

Change-Id: I4a8faf9805f940735af4fb722265fe0d7bf0edda
Reviewed-on: https://chromium-review.googlesource.com/725100Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Chris Watkins <watk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509696}

third_party/WebKit/Source/platform/heap/BlinkGCAPIReference.md

@@ -373,8 +373,6 @@ The function implementation must contain:
 
 It is recommended that the delegation call, if any, is put at the end of a tracing method.
 
-If the class does not contain any on-heap object, the tracing method is not needed.
-
 If you want to define your tracing method inline or need to have your tracing method polymorphic, you can use the
 following variants of the tracing macros: