Fix race when drawing video frames to 2d canvas.

Without this change there are possible races in GPU resource access.
Most notably PaintCanvasVideoRenderer::Paint flushes the destination
canvas after having drawn a video frame into it because the source
image buffer's content is ephemeral and is not protected by any
locking or synchronization mechanisms.  Prior to this change, the
call to PaintCanvas::flush in PaintCanvasVideoRenderer::Paint was
not effective because RecordPaintCanvas::flush was a no-op. With
this change, the flush request gets recorded so that calling code can
take care of flushing.

BUG=1254747

Change-Id: I2a99fb75f0a29d49079bba92c8f121a6c3005bcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3231460
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Juanmi Huertas <juanmihd@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#940884}

• cc/paint/paint_canvas.h[diff]
• cc/paint/record_paint_canvas.cc[diff]
• cc/paint/record_paint_canvas.h[diff]
• cc/paint/skia_paint_canvas.cc[diff]
• cc/paint/skia_paint_canvas.h[diff]
• third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.cc[diff]
• third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.h[diff]
• third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc[diff]
• third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.h[diff]
• third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d_test.cc[diff]
• third_party/blink/renderer/modules/canvas/offscreencanvas2d/offscreen_canvas_rendering_context_2d.cc[diff]
• third_party/blink/renderer/modules/canvas/offscreencanvas2d/offscreen_canvas_rendering_context_2d.h[diff]
• third_party/blink/renderer/modules/csspaint/paint_rendering_context_2d.h[diff]
• third_party/blink/renderer/platform/graphics/canvas_2d_layer_bridge.cc[diff]
• third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc[diff]
• third_party/blink/renderer/platform/graphics/canvas_resource_provider.h[diff]
• third_party/blink/renderer/platform/graphics/test/mock_paint_canvas.h[diff]
• third_party/blink/tools/blinkpy/presubmit/audit_non_blink_usage.py[diff]