GitLab

The Views implementation of SadTab supports two big things that were
missing in the Mac implementation:

1. The reload button turns into a feedback button on repeated crashes.
2. Histograms for tracking different kinds of crashes.

This moves that logic up into the base SadTab, so that both
implementations can use it.

It also adds new histograms to count when a sad tab with a reload button
or feedback button is displayed, and clicks on the button or help link.

Other changes:

- HISTOGRAM_ENUMERATION_WITH_FLAG uses a new base::underlying_value()
  function to cast scoped enums to their underlying integral values.
  (Unlike classic enums, they don't implicitly convert).

- The Show() and Close() methods of SadTab are gone. Show() was always
  called right after construction, and Close() was always called right
  before destruction, so I merged them with the ctor+dtor.

- SadTabController is gone.

- SadTabView no longer gets a reference to the WebContents; SadTabCocoa
  is now responsible for sizing it and adding it to the view hierarchy.

- The help link text no longer selects when you right click it.

BUG=623690
TEST=Trigger sad tabs on each platform either with chrome://crash or by
killing processes in the task manager. Check that the first sad tab
shows a reload button and subsequent sad tabs show a feedback button.
Otherwise, the look and behavior of the sad tab should be the same.

Review-Url: https://codereview.chromium.org/2261793002
Cr-Commit-Position: refs/heads/master@{#417606}