Blink: Have OffscreenCanvasRC2D::WritePixels call FlushRecording()

OffscreenCanvasRenderingContext2D::WritePixels() currently calls
FinalizeFrame(). This method does the following:
1. Calls GetOrCreateResourceProvider(), short-circuiting out if the
   method returns null
2. Short-circuits out if Host() returns null
3. Calls Host()->FlushRecording()

In this context, (1) and (2) are unnecessary:

* WritePixels() makes the same GetOrCreateResourceProvider() call and
  short-circuits out if it returns null just before calling
  FinalizeFrame(). GetOrCreateResourceProvider() is idempotent if
  called multiple times consecutively.
* WritePixels() calls Host() without null-checking directly below (and
  indeed, GetOrCreateResourceProvider() will return null if Host()
  returns null)

This CL replaces the FinalizeFrame() call with a direct call to
FlushRecording(). In addition to being simpler, this aligns the
structure with that of Canvas2DLayerBridge::WritePixels() - a step
toward our goal of moving the two impls into one common implementation
in BaseRenderingContext2D.

Bug: 348445884
Change-Id: I3269a334f1a1b2fe23e554cdb9615c7d3753a1a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5816871
Reviewed-by: Jean-Philippe Gravel <jpgravel@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1347863}