fix(VTT): Fix spacing between text lines

lib/text/ui_text_displayer.js

@@ -474,6 +474,7 @@ shaka.text.UITextDisplayer = class {
       wrapper = shaka.util.Dom.createHTMLElement('span');
       wrapper.classList.add('shaka-text-wrapper');
       wrapper.style.backgroundColor = cue.backgroundColor;
+      wrapper.style.lineHeight = 'normal';
       cueElement.appendChild(wrapper);
     }
 

test/text/web_vtt_layout_integration.js

@@ -265,7 +265,6 @@ filterDescribe('WebVTT layout', shaka.test.TextLayoutTests.supported, () => {
       await helper.checkScreenshot('line-85-position-50-size-63');
     });
 
-    // FIXME: UI version is slightly wrong: gaps between lines when bold
     it('bold long', async () => {
       parseAndDisplay([
         'WEBVTT\n',
@@ -278,7 +277,6 @@ filterDescribe('WebVTT layout', shaka.test.TextLayoutTests.supported, () => {
       await helper.checkScreenshot('bold-long');
     });
 
-    // FIXME: UI version is slightly wrong: gaps between lines when italic
     it('italic long', async () => {
       parseAndDisplay([
         'WEBVTT\n',
@@ -291,7 +289,6 @@ filterDescribe('WebVTT layout', shaka.test.TextLayoutTests.supported, () => {
       await helper.checkScreenshot('italic-long');
     });
 
-    // FIXME: UI version is slightly wrong: gaps between lines when italic
     it('underline long', async () => {
       parseAndDisplay([
         'WEBVTT\n',