[SVG] Prevent spurious SVGImageElement loader updates

SVGImageElement needs to update its loader after document insertion for
base URIs to work properly.

Currently, we are calling ImageLoader::updateFromElement()
indiscriminately when inserted into the document. But this can cause
spurious image reloads on element re-inserts, with visible repaint side
effects for local (uncached) resources.

The CL changes SVGImageElement::insertedInto() to only update its image
loader when needed.

R=pdr@chromium.org
BUG=346486

Review URL: https://codereview.chromium.org/177153003

git-svn-id: svn://svn.chromium.org/blink/trunk@167729 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[FastTextAutosizer] Fix crash due to no blocks returning from getTentativeClusterRoots

This patch fixes a crash in FastTextAutosizer::getSupercluster where
m_fingerprintMapper.get(block) can return a fingerprint but
m_fingerprintMapper.getTentativeClusterRoots can later return null.

I'm splitting this out of https://codereview.chromium.org/167893005.
Tests will crash with https://codereview.chromium.org/167893005 applied
without this patch.

BUG=

Review URL: https://codereview.chromium.org/178693003

git-svn-id: svn://svn.chromium.org/blink/trunk@167730 bbb929c8-8fbe-4397-9dbb-9b2b20218538