Remove the pointer from StringImpl

This CL removes the data pointer from StringImpl, reducing
sizeof(StringImpl) from 24 bytes to 12 bytes.  We now always
store the character data after the StringImpl object itself,
which means we don't need a pointer to keep track of it.

This CL saves 120 kB of memory on typical web pages (which
have ~10k strings).

R=eseidel
BUG=262320

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=155309

Review URL: https://chromiumcodereview.appspot.com/21437003

git-svn-id: svn://svn.chromium.org/blink/trunk@155404 bbb929c8-8fbe-4397-9dbb-9b2b20218538