Add Tracing.requestMemoryDump method to DevTools API (protocol)
Add Tracing.requestMemoryDump method to DevTools API (protocol)

This method will allow profiling memory tools such as Telemetry to
request memory dumps.

Proposal:
https://docs.google.com/document/d/190-URzSjfsiXNeyke86z85-5ctuqsC_RoX1J1uJTwIQ/edit?usp=sharing

Tracing handler change (Chromium side):
https://codereview.chromium.org/1232313004/

BUG=505826

Review URL: https://codereview.chromium.org/1235393005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199571 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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