Update Windows build instructions for VS 2015
Update Windows build instructions for VS 2015

Now that VS 2015 is the only supported compiler this updates the Windows
build instructions to remove all references to VS 2013.

This also adds back the step numbers for steps 3 and 4, which improves
the structure and resolves the dangling references to these steps.

The recommend .sln file to load was changed from all.sln to chrome.sln.

The block indenting for installing VS 2015 was removed. The original
goal was, I believe, to indent that section *and* the Google employees
section. This never happened and the indenting never shows up reasonably
so I'm removing it.

I tested the changes by viewing them at http://dillinger.io/

BUG=440500

Review URL: https://codereview.chromium.org/1879233009

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#387652}

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