Attach (recommended) or Link to PDF file here:
[298865.pdf](https://github.com/mozilla/pdf.js/files/2753236/298865.pdf)


Configuration:
- Web browser and its version:chrome 71.0.3578.98
- Operating system and its version:mac os 10.12.6
- PDF.js version:I use online demo in github
- Is a browser extension:no

Steps to reproduce the problem:
1. search "中华" or "电力" or so on,the highlight position is not right

What is the expected behavior? (add screenshot)
![image](https://user-images.githubusercontent.com/10000986/51087378-7eff5100-178d-11e9-991a-005d77b82f31.png)

What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/10000986/51087374-6ee77180-178d-11e9-9707-281924f95fd0.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):
https://mozilla.github.io/pdf.js/web/viewer.html
const capability = createPromiseCapability();
    let styleCache = Object.create(null);
    
    const canvas = this._document.createElement("canvas");
    canvas.height = canvas.width = DEFAULT_FONT_SIZE;
    if (
      typeof PDFJSDev === "undefined" ||
      PDFJSDev.test("MOZCENTRAL || GENERIC")
    ) {
      canvas.mozOpaque = true;
    }
    this._layoutTextCtx = canvas.getContext("2d", { alpha: false });
    if (this._textContent) {
      const textItems = this._textContent.items;
      const textStyles = this._textContent.styles;
      this._processItems(textItems, textStyles);
      capability.resolve();
    } else if (this._textContentStream) {
      const pump = () => {
        this._reader.read().then(({ value, done }) => {
          if (done) {
            capability.resolve();
            return;
          }