DevTools: Lazily compute the TreeOutline selection margin on mousedown
DevTools: Lazily compute the TreeOutline selection margin on mousedown

Bug: 843951
Change-Id: I4e71d3745ce214f467e86a338aa03ef630a2f1b0
Reviewed-on: https://chromium-review.googlesource.com/1068201
Commit-Queue: Joel Einbinder <bab755abe9fee48fe544272e55f73517d4e4a725@chromium.org>
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#562666}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e8591c53a16ed870806c419df59046be8da4d3ae
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