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This patch adds support of left/right floats in LayoutNG code.

Things that are not supported yet:
 - top aligning floats
 - clearing
 - child blocks in the same formatting formatting context

BUG=635619
TEST=virtual/layout_ng/fast/block/float, NGBlockLayoutAlgorithmTest::PositionFloatFragments

Review-Url: https://chromiumcodereview.appspot.com/2435803005
Cr-Commit-Position: refs/heads/master@{#426982}