Generate toCSSFooValue for CSS*GradientValues

As a step to use toCSSFooValue(), this cl generates toCSSGradientValue,
toCSSLinearGradientValue and toCSSRadialGradientValue. To generate them,
isLinearGradient(), isRadialGradient() need to have *Value* as a postfix.

To be consistent with other isFooValue(), they're moved from CSSGradientValue
to CSSValue.

Generate toCSSFooValue() for CSSGradiendValue, CSSLinearGradientValue and CSSRadialGradientValue

BUG=N/A

Review URL: https://codereview.chromium.org/25044003

git-svn-id: svn://svn.chromium.org/blink/trunk@158596 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: add nullable attributes

BUG=239771

Review URL: https://codereview.chromium.org/25429002

git-svn-id: svn://svn.chromium.org/blink/trunk@158597 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Rename confusing float methods on RenderBlock[Flow]

clearFloats clears the floating object set on RenderBlock and rebuilds it
from intruding floats. newLine performs the CSS version of float clearing.
I'm renaming newLine to clearFloats (which matches clearFloatsIfNeeded)
and the current version of clearFloats to rebuildFloatingObjectSetFromIntrudingFloats.

BUG=302060

Review URL: https://codereview.chromium.org/25346004

git-svn-id: svn://svn.chromium.org/blink/trunk@158598 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Support animation of stroke-opacity

Note that the existing implementation fails to clamp animated values to the
correct range. Behavior under web-animations-css matches Gecko.

BUG=257591

Review URL: https://codereview.chromium.org/25184003

git-svn-id: svn://svn.chromium.org/blink/trunk@158599 bbb929c8-8fbe-4397-9dbb-9b2b20218538
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