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In some cases, and on user's machines per bug reports, writev() performs
a short write. The child process receives an invalid profile and tabs
fail to launch. This switches to using a sequence of write() calls to
send the buffer length, and then send the buffer in a loop.

Bug: 801889
Change-Id: Ib1f4c57c61d3ce68b875b347de480f514100d3c2
Reviewed-on: https://chromium-review.googlesource.com/920765Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537360}