Part 3 - Adjust GetLogicalSkipSides() to check non-fluid continuations.

Frames split by column-span are linked together by non-fluid
continuations. We want GetLogicalSkipSides() to recognize that. The
documentation in nsIFrame::GetLogicalSkipSides() already
uses *continuation* rather than *in-flow*, so no need to adjust the
wording.

However, after patching GetLogicalSkipSides(), the first column-content
after a column-span split starts to skip the block-start side because it
has a previous continuation. We want the content in the first column to
consider applying its block-start margin. Hence the modification in
BlockReflowInput's constructor. (Note that the first column-content is
always a block-start margin-root, which is fixed in the previous part.)
This is necessary to avoid breaking
testing/web-platform/tests/css/css-multicol/multicol-span-all-margin-bottom-001.xht

In multicol-span-all-children-height-006.html, add "margin-top: 1em;" to
the container to expose the bug if we still check GetPrevInFlow() rather
than GetPrevContinuation() when computing applyBStartMargin in
nsBlockFrame::ReflowBlockFrame().

Differential Revision: https://phabricator.services.mozilla.com/D43905

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1523582
gecko-commit: be8883d72832f83fab1f4bb4bb1c1f5adfa59070
gecko-integration-branch: autoland
gecko-reviewers: dbaron

css/css-multicol/multicol-span-all-children-height-006-ref.html

@@ -29,7 +29,7 @@
   </style>
 
   <article>
-    <div class="container" style="border-bottom: none; height: 200px;">
+    <div class="container" style="border-bottom: none; height: 200px; margin-top: 1em;">
       <div class="block">block1</div>
     </div>
   </article>

css/css-multicol/multicol-span-all-children-height-006.html

@@ -18,6 +18,7 @@
     height: 250px;
     background-color: pink;
     border: 20px solid purple;
+    margin-top: 1em;
   }
   div.block {
     width: 100px;