This patch adds support for minidump generation in headless mode. This
is controlled by two new browser settings:

- SetCrashReporterEnabled: Turns crash reporter on or off. Off by
  default.
- SetCrashDumpsDir: Controls where crash dumps are written. Uses the
  directory of the executable by default.

Headless Shell is also modified to accept the equivalent command line
flags: --enable-crash-reporter and --crash-dumps-dir. Note that we don't
enable crash dumps automatically because we currently can't determine
whether the user has opted into metrics reporting.

In official builds, the generated minidumps are also uploaded
automatically. This can be disabled either with --disable-breakpad or by
setting the CHROME_HEADLESS environment variable to indicate an
unattended testing mode (as with regular Chrome).

Design doc:
https://docs.google.com/document/d/1l6AGOOBLk99PaAKoZQW_DVhM8FQ6Fut27lD938CRbTM/edit#

BUG=691507

Review-Url: https://codereview.chromium.org/2693943004
Cr-Commit-Position: refs/heads/master@{#450655}