Remove searchableIndexIntroduction(). Use Locale::queryString() instead.

We always use the default locale for this because
 - We can't use Element::locale in processIsindexStartTagForInBody.
 - We need some amount of code to obtain appropriate lang attribute
   value here, and it needs runtime cost.
 - However, <isindex> is an obsolete feature. It's not reasonable to pay
   the cost to it.

This CL adds ASSERT(isMainThread()) in Locale::defaultLocale() because
Locale class is not ready for multi-threading and a part of our HTML parser
runs on non-main thread. HTMLTreeBuilder runs on the main thread for
now.

BUG=298020
TEST=none; no behavior changes.
R=keishi@chromium.org

Review URL: https://codereview.chromium.org/25431002

git-svn-id: svn://svn.chromium.org/blink/trunk@158600 bbb929c8-8fbe-4397-9dbb-9b2b20218538