Safely iterate over MediaStreamSource observers.

Safely iterate over MediaStreamSource observers.

When changing the ready state of this object, the resultant dispatching of
events by its observers may extend the observer set. Take a snapshot of
the observers, so as to be able safely iterate over it across additions.

R=
BUG=602273
Review URL: https://codereview.chromium.org/1885053002

Cr-Commit-Position: refs/heads/master@{#387243}
(cherry picked from commit bc3ebdc42cd0b29fd770213d02a99a81b84dc69c)

Committed: https://chromium.googlesource.com/chromium/src/+/cc916de6955699b06e38ed659805c1fc3904126c

[sof, 2016-04-18 18:54:08]

Description was changed from

==========
Safely iterate over MediaStreamSource observers.

When changing the ready state of this object, the resultant dispatching of
events by its observers may extend the observer set. Take a snapshot of
the observers, so as to be able safely iterate over it across additions.

R=
BUG=602273

Review URL: https://codereview.chromium.org/1885053002

Cr-Commit-Position: refs/heads/master@{#387243}
(cherry picked from commit bc3ebdc42cd0b29fd770213d02a99a81b84dc69c)
==========

to

==========
Safely iterate over MediaStreamSource observers.

When changing the ready state of this object, the resultant dispatching of
events by its observers may extend the observer set. Take a snapshot of
the observers, so as to be able safely iterate over it across additions.

R=
BUG=602273

Review URL: https://codereview.chromium.org/1885053002

Cr-Commit-Position: refs/heads/master@{#387243}
(cherry picked from commit bc3ebdc42cd0b29fd770213d02a99a81b84dc69c)

Committed:
https://chromium.googlesource.com/chromium/src/+/cc916de6955699b06e38ed659805...
==========

[sof, 2016-04-18 18:54:10]

Committed patchset #1 (id:1) manually as
cc916de6955699b06e38ed659805c1fc3904126c.