Rename [Web]PageSerializer[Test|Client|Impl] to ...FrameSerializer...

After refactoring needed to support OOPIFs in save-page-as feature, the
html serializers in blink serialize frames, not pages and therefore
their old names (WebPageSerializer, PageSerializer, etc) do not apply
anymore.  Therefore this CL renames PageSerializer to FrameSerializer
where applicable.

Note that this rename not only covers PageSerializer class, but also
WebPageSerializer, WebPageSerializerClient, PageSerializerTest, test
data files, etc.

Note that FrameSerializer is more correct than HtmlSerializer, because
- FrameSerializer also serializes subresources (i.e. images)
- Even though WebFrameSerializerImpl deals only with HTML documents,
  for now it might make sense to leave it named as WebFrameSerializerImpl:
  - it still takes a Frame as input
  - it implements part of the public WebFrameSerializer class
    (and therefore it is desirable to have a similar name here as well)

BUG=538766,526786

Review URL: https://codereview.chromium.org/1541463002

Cr-Commit-Position: refs/heads/master@{#367227}

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