[CSS Grid Layout]	ASSERTION FAILED: repetitions > 0

If the author specifies an absurdly large number of repetitions it might
happen that the double value contained in the CSSParserValue does not fit in
a size_t, so it is clamped down to 0 during the type casting.

From now on we limit the value of repetitions to kGridMaxTracks (1 million)
before casting the double value to a size_t.

BUG=425001

Review URL: https://codereview.chromium.org/645793005

git-svn-id: svn://svn.chromium.org/blink/trunk@184154 bbb929c8-8fbe-4397-9dbb-9b2b20218538