Make fast/dom/Range/range-expand.html match its intention

The layout test has a "multiline" test case which isn't really multiline,
as the linebreak characters in HTML are turned into whitespaces.

This patch changes the <p> element into <pre> so that the linebreak
characters are really breaking text into lines, and the test case is a
multiline test case.

This patch is a preparation for changing StartOfSentence() to use
TextOffsetMapping (crrev.com/c/1335959).

Bug: 778507
Change-Id: I77cd48eb13a1ff442e7ecbe1c71819055bccf078
Reviewed-on: https://chromium-review.googlesource.com/c/1336785
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608263}

third_party/WebKit/LayoutTests/fast/dom/Range/range-expand.html

@@ -64,20 +64,20 @@ function runTests() {
     assert_equals(actual, 'Roll the mouse around the page. The coordinates\x0Aof the mouse pointer are currently atop an element');
   }, 'sentence7');
 
-  // test 8 - Sentence does not go upstream?
+  // test 8
   test(() => {
     const actual = expandRangeString('multilineSentence', 0, 'multilineSentence', 0, 'sentence', false);
-    assert_equals(actual, 'Now, a sentence\x0Ais divided into\x0Amultiple lines.');
+    assert_equals(actual, 'Now, a sentence\n');
   }, 'sentence8');
 
   test(() => {
     const actual = expandRangeString('multilineSentence', 1, 'multilineSentence', 5, 'sentence', true);
-    assert_equals(actual, 'Now, a sentence\x0Ais divided into\x0Amultiple lines.');
+    assert_equals(actual, 'Now, a sentence\n');
   }, 'sentence9');
 
   test(() => {
     const actual = expandRangeString('multilineSentence', 20, 'multilineSentence', 20, 'sentence', true);
-    assert_equals(actual, 'is divided into\x0Amultiple lines.');
+    assert_equals(actual, 'is divided into\n');
   }, 'sentence10');
 
   test(() => {
@@ -221,10 +221,10 @@ function runTests() {
 <p>This is the begin of a block. A block is a collection of sentences. Each sentence begins with capital
 letter and ends with a punctuation.
 </p>
-<p id="multilineSentence">Now, a sentence
+<pre id="multilineSentence">Now, a sentence
 is divided into
 multiple lines.
-</p>
+</pre>
 <p id="instructions">Roll the mouse around the page. The coordinates
 of the mouse pointer are currently atop an element<br id="mybr">whose ID 
 is:"<span id="myspan" style="font-weight:bold"></span>".</p>