[Extensions UI] Initially disabled OK button for extension install prompts and enable them after a 500 ms time period.

BUG=394518

Review-Url: https://codereview.chromium.org/2716353003
Cr-Commit-Position: refs/heads/master@{#461933}
 void ExtensionInstallDialogView::UpdateInstallResultHistogram(bool accepted)
     const {
   if (prompt_->type() == ExtensionInstallPrompt::INSTALL_PROMPT)
    UMA_HISTOGRAM_BOOLEAN("Extensions.InstallPrompt.Accepted", accepted);
}

 void ExtensionInstallDialogView::UpdateInstallResultHistogram(bool accepted)
     const {
   if (prompt_->type() == ExtensionInstallPrompt::INSTALL_PROMPT)
    UMA_HISTOGRAM_BOOLEAN("Extensions.InstallPrompt.Accepted", accepted);
}
