Adjustments in InlineTextBox.

* updateGraphicsContext and correctedTextColor are not used anywhere
  else, so they are made static functions
* correctedTextColor sheds unnecessary generality (backgroundColor is
  always Color::white)
* InlineTextBox::textRenderer is inlined at its declaration, which
  is more concise and easier to find

BUG=388946

Review URL: https://codereview.chromium.org/361253007

git-svn-id: svn://svn.chromium.org/blink/trunk@177796 bbb929c8-8fbe-4397-9dbb-9b2b20218538