Add node IDs to PaintCanvas/GraphicsContext for accessible PDFs.

In order for Chrome to output tagged PDFs, it needs content painted
by Blink to be tagged with its associated DOM node. Text is already
marked with its DOMNodeID. This patch extends that to tag
arbitrary drawing in GraphicsContext.

For background, see this design doc:
https://docs.google.com/document/d/1ku6QNtAHEqVnRGqFzpmlMe1fowkjCX9miHj8Kx1cFlk/edit#

Bug: 607777
Change-Id: Iec5cab6aa22c79ca3ce4db681164a97541264506
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1927975
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747814}

• cc/paint/paint_canvas.h[diff]
• cc/paint/paint_op_buffer.cc[diff]
• cc/paint/paint_op_buffer.h[diff]
• cc/paint/paint_op_buffer_unittest.cc[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]
• cc/paint/solid_color_analyzer.cc[diff]
• cc/test/paint_op_helper.h[diff]
• third_party/blink/renderer/core/layout/layout_text.cc[diff]
• third_party/blink/renderer/core/layout/line/inline_box.cc[diff]
• third_party/blink/renderer/core/layout/line/inline_box.h[diff]
• third_party/blink/renderer/platform/graphics/graphics_context.cc[diff]
• third_party/blink/renderer/platform/graphics/graphics_context.h[diff]
• third_party/blink/renderer/platform/graphics/paint/drawing_recorder.cc[diff]
• third_party/blink/renderer/platform/graphics/paint/drawing_recorder.h[diff]
• third_party/blink/renderer/platform/graphics/test/mock_paint_canvas.h[diff]