Support scientific notation in CSS numbers/percentages/dimensions

Previously, scientific notation (scinot) was only supported for SVG
presentation attributes through a special tokenizer quirk. Since scinot
has now been added to the CSS syntax spec [1][2][3] and is already
supported by Gecko [4] and IE(11).
To harmonize, the call to parseSVGNumber(...) is replaced with a call to
charactersToDouble(...). This results in a small behavioral change, since
the former would reject numbers that were too large. These will now parse
as "Infinity" [5].
Since this removes the last caller of isSVGNumberParsingEnabledForMode()
that function is removed as well.

[1] http://dev.w3.org/csswg/css-syntax/#number-token-diagram
[2] http://lists.w3.org/Archives/Public/www-style/2012Aug/0900.html
[3] http://lists.w3.org/Archives/Public/www-style/2013Apr/0428.html
[4] https://bugzilla.mozilla.org/show_bug.cgi?id=964529
[5] Tested browsers were not entirely consistent in their treatment of
    infinities (Gecko: Infinity, IE: FLT_MAX).

BUG=388161
BUG=344645

Review URL: https://codereview.chromium.org/545173003

git-svn-id: svn://svn.chromium.org/blink/trunk@181560 bbb929c8-8fbe-4397-9dbb-9b2b20218538