Fix crash when user closes window prior to the "Confirm Install" prompt showing.
This CL moves ExtensionInstallPrompt::ShowParams to its own class. The new class
observes the content::WebContents and gfx::NativeWindow for their destruction.
If the window is closed prior to the "Confirm Install" prompt showing, the
prompt is shown as a non-modal free-standing dialog.
BUG=422814
TEST=ExtensionInstallPromptBrowserTest.*
Review URL: https://codereview.chromium.org/662073002
Cr-Commit-Position: refs/heads/master@{#301674}
26 files changed
