Make constraint validation algorithm for minlength attribute match re…

…ality

Per http://caniuse.com/#feat=input-minlength ,
only Blink currently implements minlength, and it implements it similar to
maxlength, which means it requires the user to edit the value in order for it
to be considered "too short".
So this commit copies the dirtiness and "last edited by the user" conditions
from maxlength's constraint validation algorithm
into minlength's constraint validation algorithm.

Closes #174.

source

@@ -52524,7 +52524,9 @@ out of 233&#x2009;257&#x2009;824 bytes available&lt;/meter>&lt;/p></pre>
   <div w-nodev>
 
   <p><strong>Constraint validation</strong>: If an element has a <span>minimum allowed value
-  length</span>, its <span data-x="concept-fe-value">value</span> is not the empty string, and the
+  length</span>, its <var>dirty value flag</var> is true, its <span
+  data-x="concept-fe-value">value</span> was last changed by a user edit (as opposed to a change made
+  by a script), its <span data-x="concept-fe-value">value</span> is not the empty string, and the
   <span>code-unit length</span> of the element's <span data-x="concept-fe-value">value</span> is less
   than the element's <span>minimum allowed value length</span>, then the element is <span>suffering
   from being too short</span>.</p>