Add isBrowser as a property of sheet so that it can be manually set.

[tkh44]

Why?
You can get get the size of the css output in storybook or any other demo.

const renderedCssSize = (Component = () => <div />) => {
  sheet.isBrowser = false;
  sheet.flush();
  const { css } = extractCritical(() =>
    ReactDOMServer.renderToString(
      <ThemeProvider theme={theme}>
        <Component />
      </ThemeProvider>,
    ),
  );
  sheet.isBrowser = true;
  return css.length;
};

Checklist:

• Documentation N/A
• Tests
• Code complete

[codecov]

Codecov Report

Merging #377 into master will not change coverage.
The diff coverage is 100%.

Impacted Files	Coverage Δ
packages/emotion/src/sheet.js	84.44% <100%> (ø)	⬆️