Add support for SourceBuffer.appendStream()

BUG=249431
TEST=LayoutTests/http/tests/media/media-source/mediasource-append-stream.html

Review URL: https://chromiumcodereview.appspot.com/22765002

git-svn-id: svn://svn.chromium.org/blink/trunk@158162 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Make runtime model less noisy when evaluate is not possible.

This patch removes console error message when evaluate is not possible because execution context is not available anymore.
This happens in case when there are some watch expressions added in scripts panel and inspected page is reloaded. 

BUG=

Review URL: https://chromiumcodereview.appspot.com/23498032

git-svn-id: svn://svn.chromium.org/blink/trunk@158163 bbb929c8-8fbe-4397-9dbb-9b2b20218538