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The Windows build instructions for Chromium leads many people to assume
that the --ide command is required. This change makes it clearer that it
is optional, and also expands on how to use the alternatives.

This also changes the recommend invocation of "gclient sync" to add
"-D".

Tested with https://markdownlivepreview.com/

Change-Id: Ia341f3021c320a4cf7a7071fb96f0464078baf3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444450
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813438}