Provide a method to remove inserted style sheet

InsertStyleSheet() returns sequence id. And RemoveInsertedStyleSheet()
removes the inserted style sheet with its id.

TEST=webkit_unit_tests --gtest_fiter=StyleEngineTest.AnalyzedInject, WebDocumentTest.InsertAndRemoveStyleSheet
BUG=608854

Review-Url: https://codereview.chromium.org/2835183002
Cr-Commit-Position: refs/heads/master@{#474919}
7 files changed