Split ImageResourceClient into ResourceClient and ImageResourceObserver [1/2]

We are planning to move ResourceClient to on-heap, but LayoutObject (an
ImageResourceClient subclass) is hard to move to on-heap.

This series of CLs splits ImageResourceClient into
- ResourceClient (notified via Resource, to be moved to Oilpan heap) and
- ImageResourceObserver (notified via ImageObserver, to remain non-heap)
and makes LayoutObject to be ImageResourceObserver but not ResourceClient.

This is the [1/2] CL that
- Splits ImageResourceClient into ResourceClient and ImageResourceObserver,
- Makes LayoutObject and all of its subclasses (except for LayoutImage and its
  subclasses) to be ImageResourceObserver but not ResourceClient.

LayoutImage will be made non-ResourceClient by the [2/2] CL:
https://codereview.chromium.org/1728313003/.

Original CL: https://codereview.chromium.org/1697713002/ by Nate Chapin.

In addition to Nate's original CL (Patch Set 1), this CL:
- Fixes layout test (Patch Set 2).
- Reuses ResourceClientWalker to iterate ImageResourceObserver (Patch Set 3).
- Handles ResourceClient and ImageResourceObserver more similarly (Patch Set 4).
- Renames methods to reflect whether they are for both ResourceClient and
  ImageResourceObserver (Patch Set 6).

BUG=587663

Review URL: https://codereview.chromium.org/1706083002

Cr-Commit-Position: refs/heads/master@{#383425}

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