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CL:1924341 limited the horizontal scroll of the directorytree to 0, to
resolve issue 1025581, and needs a test of this behavior.

Create a nested tree of test folders. Expand the directory tree to the
last folder, verify the navigation, and that the directory tree is not
horizontally scrolled (directorytree.scrollLeft should be 0). The test
passes with CL:1924341 and fails before that CL (good).

Tbr: anyone
Test: browser_tests --gtest_filter="*TreeExpandHorizontal*"
Bug: 1029019
Change-Id: I609b06ddfc3d18cf24cf5b5c3e4e118c06989c35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942667Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720078}