Write README.chromium in 'wb' mode to avoid CRs
Write README.chromium in 'wb' mode to avoid CRs

If crashpad\update.py is run on Windows then README.chromium shows up
as having all lines modified. This is because it was opened in 'w' mode
instead of 'wb' mode. Fixing this makes it easier to run README.chromium
on Windows.

Review URL: https://codereview.chromium.org/1531503002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365335}

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