Improve speed of WebIconSizesParser

Reduce time complexity from O(n^2) to O(n)

Bug: chromium:855272, chromium:1009098
Change-Id: I9365c7ae7135ec694a5b83ef47674461fc7f353e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854104Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705091}

third_party/blink/renderer/platform/exported/web_icon_sizes_parser.cc

@@ -65,7 +65,7 @@ WebVector<WebSize> WebIconSizesParser::ParseIconSizes(
       break;
 
     // See if the current size is "any".
-    if (sizes_string.FindIgnoringCase("any", i) == i &&
+    if (sizes_string.Substring(i, 3).StartsWithIgnoringCase("any") &&
         (i + 3 == length || IsWhitespace(sizes_string[i + 3]))) {
       icon_sizes.push_back(WebSize(0, 0));
       i = i + 3;