add trace events to XMLDocumentParser
add trace events to XMLDocumentParser

XMLDocumentParser is expected to be slow and we plan to use HTMLDocumentParser as much as possbile.
These tracing events measuring time spent inside parser would help us understand its potential impact.

BUG=None

Review URL: https://codereview.chromium.org/368243008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@177515 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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