Fix a race condition in InputTypeNames::image().

This function is not thread-safe, but TokenPreloadScanner::StartTagScanner::
processAttribute() calls it in a parser thread. if InputTypeNames::image() is
called concurrently, one of them can return a null reference.

BUG=318989
TEST=none; This bug is very timing-depedent.

Review URL: https://codereview.chromium.org/68103017

git-svn-id: svn://svn.chromium.org/blink/trunk@161989 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add timeout expectation for fast/line-grid/line-align-left-edges.html

TBR=yurys
BUG=313659

Review URL: https://codereview.chromium.org/72253002

git-svn-id: svn://svn.chromium.org/blink/trunk@161990 bbb929c8-8fbe-4397-9dbb-9b2b20218538