Consolidate javascript_dialog_manager.h and javascript_dialog_manager_impl.h

* Renamed JavaScriptDialgoManagerImpl to JavaScriptDialogManager.
* Add namespace app_modal_dialogs
* Add namespace javascript_dialog_extensions_client
* Shortened InstallJavaScriptDialogExtensionsClient to InstallClient to fit the 80 chars limit.

BUG=410499
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Review URL: https://codereview.chromium.org/688853005

Cr-Commit-Position: refs/heads/master@{#304903}
diff --git a/components/app_modal/native_app_modal_dialog.h b/components/app_modal/native_app_modal_dialog.h
index 192619a..e41ff48c 100644
--- a/components/app_modal/native_app_modal_dialog.h
+++ b/components/app_modal/native_app_modal_dialog.h
@@ -7,6 +7,8 @@
 
 #include "ui/gfx/native_widget_types.h"
 
+namespace app_modal {
+
 class JavaScriptAppModalDialog;
 
 class NativeAppModalDialog {
@@ -29,4 +31,6 @@
   virtual void CancelAppModalDialog() = 0;
 };
 
+}  // namespace app_modal
+
 #endif  // COMPONENTS_APP_MODAL_NATIVE_APP_MODAL_DIALOG_H_