Make WebPageSerializerImpl to escape URL attribute values in result.

This patch makes |WebPageSerializerImpl| to escape URL attribute values rather
than directly output URL attribute values into result.

BUG=542054
TEST=webkit_unit_tests --gtest_filter=WebPageSerializerTest.URLAttributeValues

Review URL: https://codereview.chromium.org/1398453005

Cr-Commit-Position: refs/heads/master@{#353712}
(cherry picked from commit b770d85e37b2d0e248f04cf20606a2f3871ef039)

Review URL: https://codereview.chromium.org/1487273003 .

Cr-Commit-Position: refs/branch-heads/2526@{#496}
Cr-Branched-From: cb947c0153db0ec02a8abbcb3ca086d88bf6006f-refs/heads/master@{#352221}
3 files changed