feat(Page): kill the page.plainText method (#994)

The page.plainText is confusing: it's unclear what kind of text it
returns, textContent or innerText. It's also easily polyfillable and
doesn't seem to be used.

BREAKING CHANGE: the page.plainText is not existing any more.
Instead, use `page.evaluate(() => document.body.innerText)`.

docs/api.md

@@ -58,7 +58,6 @@
     + [page.mainFrame()](#pagemainframe)
     + [page.mouse](#pagemouse)
     + [page.pdf(options)](#pagepdfoptions)
-    + [page.plainText()](#pageplaintext)
     + [page.reload(options)](#pagereloadoptions)
     + [page.screenshot([options])](#pagescreenshotoptions)
     + [page.select(selector, ...values)](#pageselectselector-values)
@@ -764,9 +763,6 @@ The `format` options are:
 - `A4`: 8.27in x 11.7in
 - `A5`: 5.83in x 8.27in
 
-#### page.plainText()
-- returns:  <[Promise]<[string]>> Returns page's inner text.
-
 #### page.reload(options)
 - `options` <[Object]> Navigation parameters which might have the following properties:
   - `timeout` <[number]> Maximum navigation time in milliseconds, defaults to 30 seconds, pass `0` to disable timeout.

lib/Page.js

@@ -664,13 +664,6 @@ class Page extends EventEmitter {
     return buffer;
   }
 
-  /**
-   * @return {!Promise<string>}
-   */
-  async plainText() {
-    return this.evaluate(() =>  document.body.innerText);
-  }
-
   /**
    * @return {!Promise<string>}
    */

test/test.js

@@ -2186,13 +2186,6 @@ describe('Page', function() {
     }));
   });
 
-  describe('Page.plainText', function() {
-    it('should return the page text', SX(async function(){
-      await page.setContent('<div>the result text</div>');
-      expect(await page.plainText()).toBe('the result text');
-    }));
-  });
-
   describe('Page.title', function() {
     it('should return the page title', SX(async function(){
       await page.goto(PREFIX + '/input/button.html');