Telemetry: Only print the list of available benchmarks.

Telemetry: Only print the list of available benchmarks.

Only print the list of available benchmarks if a '--browser' argument has been specified on the command line.

On Windows, only 60 / ~130 are available, the other ones are disabled.

Here's how it look like now:

Available benchmarks for exact are:
  blink_perf.animation
  ...
  webrtc.webrtc_cases                           Obtains WebRTC metrics for a real-time video tests.
Disabled benchmarks for exact are (force run with -d):
  browsermark                                   Browsermark suite tests CSS, DOM, resize, page load, WebGL and JS.
  ...
  v8.garbage_collection_cases                   Measure V8 metrics on the garbage collection cases.
Pass --browser to list benchmarks for another browser.

BUG=434870

Review URL: https://codereview.chromium.org/743213002

Cr-Commit-Position: refs/heads/master@{#305140}