src: Merge with 'upstream/master'
assert: indicate if exception message is generated

AssertionError.generatedMessage is now true when
AssertionError.message was generated from expected and actual

Fixes #5836, #6206
buffer: check data is not null

Because it's possible for the data within a Buffer instance to be
altered after instantiation, or in case a user attempts to do something
like the following:

Buffer.prototype.fill.call({}, 10, 0, 10);

It doesn't result in a segfault.
buffer: add buf.toArrayBuffer() API
lint: fix a cpplint error

Added a NOLINT so that cpplint won't complain about some code.
src: add accessor method for debugger_running flag.

Reason: If a developer wants to implement a module that needs to know
debug mode status.

Example: We use a module process-watcher which monitors nodejs processes
for endless loops and other abnormal behaviours and kills it. We would
not want it to kill a worker if it in debug mode. To ensure that we have
a cpp module, monitr, that sends the debug mode status to process-watcher
(https://npmjs.org/package/process-watcher).
Merge branch 'upstream/master' into debug_toggle
Merge 'debug_toggle' of github.com:yahoo/node