chromedriver doesn't allow changing Object.prototype to add enumerable
properties, but this test requires setting some values on
Object.prototype.  When Object.prototype.a is set to:

  {b: {c: 'on proto'}}

chromedriver fails with:

    JavascriptErrorException: javascript error (500): Maximum call stack size exceeded
      (Session info: chrome=72.0.3626.121)

    Remote-end stacktrace:

    #0 0x563ff3a32a59 <unknown>
    #1 0x563ff39cb7f3 <unknown>
    #2 0x563ff38fcd7c <unknown>
    #3 0x563ff38ff78c <unknown>
    #4 0x563ff38ff5f7 <unknown>
    #5 0x563ff38ffbe7 <unknown>
    #6 0x563ff38fff1b <unknown>
    #7 0x563ff38a3f7a <unknown>
    #8 0x563ff3899bf2 <unknown>
    #9 0x563ff38a37b7 <unknown>
    #10 0x563ff3899ac3 <unknown>
    #11 0x563ff38782d2 <unknown>
    #12 0x563ff3879112 <unknown>
    #13 0x563ff39fe865 <unknown>
    #14 0x563ff39ff32b <unknown>
    #15 0x563ff39ff70c <unknown>
    #16 0x563ff39d940a <unknown>
    #17 0x563ff39ff997 <unknown>
    #18 0x563ff39e9947 <unknown>
    #19 0x563ff3a1a800 <unknown>
    #20 0x563ff3a3c8be <unknown>
    #21 0x7f3bf4545494 start_thread
    #22 0x7f3bf2d58a8f clone

    Ran 1 tests finished in 2.0 seconds.
      • 0 ran as expected. 0 tests skipped.
      • 1 tests had errors unexpectedly

Work around this problem by cleaning up the test environment so
Object.prototype no longer has the override by the time chromedriver
tries to inspect the test result.

While here, fix the other tests to use the t.add_cleanup() function
so they'll cleanup their test environment in case they exit in
some other way besides reaching t.done().

The underlying chromedriver issue is tracked upstream at
https://crbug.com/chromedriver/2555.

Bug: 934844
Change-Id: Id1b4ab2a908bfbc001e2a2d045eeec3ef01c24d9

Since @page border box layout objects aren't in the the layout tree, any
code that wants to walk up the tree to find the containing block will be
in for a surprise.

This would happen if percentage-based @page padding was used [1].
Recomputing padding during painting when we have already done it during
layout is rather pointless anyway. Read it out directly from the
fragment.

[1] #1 blink::LayoutBox::ContainingBlockLogicalWidthForContent()
    #2 blink::LayoutBoxModelObject::ComputedCSSPadding()
    #3 blink::LayoutBoxModelObject::PaddingTop()
    #4 blink::LayoutBoxModelObject::PaddingOutsets()
    #5 blink::BoxPainterBase::PaintFillLayer()
    #6 blink::BoxPainterBase::PaintFillLayers()
    #7 blink::BoxFragmentPainter::PaintBackground()

Bug: 40286153
Change-Id: I1e6e92c2ce1d81aab2673ec9a877eac455534102
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5526469
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300711}