Toggle diff view mode with 'm' key

Ieae237f9bb1 adds a keyboard shortcut to switch diff view modes in the
diff view. With this change, the key is changed from 'v' to 'm', support
is added to the change view as well as the diff view, and tests are
added.

Bug: Issue 8269
Change-Id: Ifeaa26bc1d6256809d58673232cf982b6faac400

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
index 8b6d82f..ae98fe9 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
@@ -798,6 +798,19 @@
       assert.equal(element._getDiffViewMode(), 'SIDE_BY_SIDE');
     });
 
+    test('_handleMKey', () => {
+      sandbox.stub(element, 'shouldSuppressKeyboardShortcut').returns(false);
+      const e = {preventDefault: () => {}};
+      // Initial state.
+      assert.equal(element._getDiffViewMode(), 'SIDE_BY_SIDE');
+
+      element._handleMKey(e);
+      assert.equal(element._getDiffViewMode(), 'UNIFIED_DIFF');
+
+      element._handleMKey(e);
+      assert.equal(element._getDiffViewMode(), 'SIDE_BY_SIDE');
+    });
+
     suite('_loadComments', () => {
       test('empty', done => {
         element._loadComments().then(() => {