Oilpan: Clear HTMLDocumentParser::m_token when the HTMLDocumentParser is detached

This CL improves tiny-innerHTML by 50%.

non-oilpan: 4.2 runs/sec
oilpan (before this CL): 3.3 runs/sec
oilpan (after this CL): 5.0 runs/sec

In non-oilpan builds, backing memory of HTMLToken::m_data (which is Vector<UChar, 256>) is reused for each innerHTML setter and thus the memory locality is good. On the other hand, in oilpan builds without this CL, the backing memory of HTMLToken::m_data is not reused until HTMLDocumentParser objects are collected by a next GC. Thus the memory locality is bad.

This CL resolves the locality problem by explicitly clearing HTMLDocumentParser::m_token when the HTMLDocumentParser gets detached.

BUG=420515

Review URL: https://codereview.chromium.org/677933002

git-svn-id: svn://svn.chromium.org/blink/trunk@184510 bbb929c8-8fbe-4397-9dbb-9b2b20218538