CSS Tokenizer: Preallocate space for parser tokens to avoid resizing
CSS Tokenizer: Preallocate space for parser tokens to avoid resizing

This patch makes the CSS Tokenizer preallocate space for length(s)/3
parser tokens at the start of tokenization. This ratio was chosen to
avoid copying parser tokens due to resizes. From a sample of top Alexa
pages, characters per token usually ranges from 3.5 to 5.

This improves our performance on css-parser-yui.html by about 12%.
Note the preallocated space is not optimised for this test, which has
a ratio of about 4 characters per token.

BUG=455064

Review URL: https://codereview.chromium.org/939073005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190533 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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