[LayoutNG] Only DCHECK that we're at FC origin if at new FC.

For fragmentation, if we have a break token, the inline coordinate
may still be non-zero at the start of a next fragmentainer, if the
block we're fragmenting has an inline-start margin.
Furthermore, even the block coordinate may be non-zero, if
clearance has been applied.

R=cbiesinger@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I998eabbbc2434574aff2e63dc7114f942c3c92c3
Reviewed-on: https://chromium-review.googlesource.com/799970
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520777}

third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-features=LayoutNG

@@ -5232,10 +5232,8 @@ crbug.com/591099 fast/loader/javascript-url-iframe-crash.html [ Crash ]
 crbug.com/591099 fast/media/mq-display-mode-fullscreen.html [ Crash ]
 crbug.com/591099 fast/multicol/anonymous-block-split-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/auto-height-forced-break-complex-margin-collapsing.html [ Failure ]
-crbug.com/591099 fast/multicol/balance-breakafter-before-table-section-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/break-after-empty-set-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/change-block-child-height.html [ Crash ]
-crbug.com/591099 fast/multicol/client-rect-overflowing-multicol-2-columns.html [ Crash ]
 crbug.com/591099 fast/multicol/clone-block-children-inline-mismatch-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/composited-relpos-in-clipped.html [ Pass ]
 crbug.com/591099 fast/multicol/constrained-content-height-with-overflow-crash.html [ Crash ]
@@ -5377,7 +5375,6 @@ crbug.com/591099 external/wpt/css/css-multicol/multicol-width-count-002.xht [ Fa
 crbug.com/591099 external/wpt/websockets/opening-handshake/005.html [ Pass ]
 crbug.com/591099 fast/multicol/input-with-overflow-second-column.html [ Failure ]
 crbug.com/591099 fast/multicol/large-padding-crash.html [ Crash ]
-crbug.com/591099 fast/multicol/layers-split-across-columns.html [ Crash ]
 crbug.com/591099 fast/multicol/margin-bottom-and-break-after.html [ Crash ]
 crbug.com/591099 fast/multicol/min-height-much-greater-than-content.html [ Failure ]
 crbug.com/591099 fast/multicol/mixed-opacity-fixed-test.html [ Pass ]
@@ -5386,7 +5383,6 @@ crbug.com/591099 fast/multicol/negative-margins-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/nested-multicol-two-spanners-dynamic.html [ Crash ]
 crbug.com/591099 fast/multicol/nested-with-spanner-and-margin-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/nested-with-spanner-inside-margins-crash.html [ Crash ]
-crbug.com/591099 fast/multicol/newmulticol/clipping.html [ Crash ]
 crbug.com/591099 fast/multicol/newmulticol/leading-margin.html [ Crash ]
 crbug.com/591099 fast/multicol/newmulticol/spanner-inside-child-crash.html [ Crash ]
 crbug.com/591099 fast/multicol/out-of-flow/abspos-auto-left-right.html [ Pass ]

third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc

@@ -256,7 +256,14 @@ scoped_refptr<NGLayoutResult> NGBlockLayoutAlgorithm::Layout() {
   if (ConstraintSpace().IsNewFormattingContext() || BreakToken()) {
     MaybeUpdateFragmentBfcOffset(input_bfc_block_offset);
     DCHECK(input_margin_strut.IsEmpty());
-    DCHECK_EQ(container_builder_.BfcOffset().value(), NGBfcOffset());
+#if DCHECK_IS_ON()
+    // If this is a new formatting context, we should definitely be at the
+    // origin here. If we're resuming at a fragmented block (that doesn't
+    // establish a new formatting context), that may not be the case,
+    // though. There may e.g. be clearance involved, or inline-start margins.
+    if (ConstraintSpace().IsNewFormattingContext())
+      DCHECK_EQ(container_builder_.BfcOffset().value(), NGBfcOffset());
+#endif
   }
 
   input_bfc_block_offset += intrinsic_block_size_;