Linux native notifications: Only escape <, >, and & characters for HTML

Canonical's notification development guidelines [1] specifies that
only '<', '>', and '&' characters should be escaped when the
notification server supports the 'body-markup' capability.

Previously, we were escaping single and double quote characters as
well.  On Cinnamon, this would cause the ' character to render as
&#39;.  This CL only escapes the recommended chracters.

[1] https://wiki.ubuntu.com/NotificationDevelopmentGuidelines#If_a_notification_uses_a_.2BIBw.3C.2BIB0_or_.2BIBw.26.2BIB0_character.2C_and_it_has_missing_or_wrong_text

BUG=676220
TBR=peter@chromium.org
CC=​thestig@chromium.org

Change-Id: I3e6f6ea60be52a9e4f406c728701f9f666362bc6
Reviewed-on: https://chromium-review.googlesource.com/688294
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505133}
2 files changed