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This change adds 4 new performance histograms for the DevTools using the
prefix 'DevTools.Launch':
- DevTools.Launch.Console
- DevTools.Launch.Elements
- DevTools.Launch.Network
- DevTools.Launch.Sources

These histograms measure the time between the DevTools window being
shown during 'DevToolsWindow::ToggleDevToolsWindow' and a point in each
tool that corresponds to the 'ready' state as seen by a user. These end
points are unique to each tool but are as follows:
- Console: At the end of the ConsolePrompt constructor
- Elements: At the end of _updateMetrics when the styles panel has
  been populated for the initially selected node
- Network: After the panel is shown
- Sources: After the NetworkNavigatorView has been loaded

These tools were chosen as they represent the most popular in terms of
usage and will allow us to identify any significant performance impact
in the time taken to launch the DevTools when these panels open by
default.

To see time captured by the histograms in terms of a perf trace for
each tool please take a look at the following screenshots:
https://imgur.com/a/rcAgQ8h

In the traces you can see that the end marker for each load is at the
end of the trace near first meaningful paint, and just as the user
will see the content of the tool render fully.

To accomplish the histograms, a new native function has been added to
the DevTools ui bindings/embedder classes called
'recordPerformanceHistogram'. Along with the plumbing and a new helper
function in the frontend javascript Host.UserMetrics file called
'panelLoaded'

When the frontend javascript loads, it will store the initial panel to
be loaded from the preferences. Once that panel has been created and
the appropriate loaded marker has been hit, the duration of the launch
will be calculated using performance.now(). That duration is then sent
to the native code which will fire the appropriate histogram via the
'UMA_HISTOGRAM_TIMES' macros.

The mechanism is similar to the existing one used for enumerated
histograms and can be expanded to fire additional performance histograms
in the future.

Also added a new test for user-metrics to ensure that the panelLoaded
function will correctly record the histograms.

Change-Id: Ib9f0bf80c651231d6d2e308bee36c1a1223f04db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506388Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: James Lissiak <jalissia@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#644407}