AW docs: upstream web page layout guide

No change to logic, only docs. This upstreams the web page layout guide,
mostly autogenerated from the google sites page (tables were converted
by hand).

I made some minor edits to the text for readability, but the content is
mostly unchanged.

This omits the "experimental data" section on the original doc because
(1) this isn't very relevant anymore since the behavior has shipped and
we don't intend to reevaluate and (2) the data was internal-only.

Fixed: 1083508
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ic1ece33a5565986235af1dbb735d35966253fe5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393006Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804429}

android_webview/docs/web-page-layout.md

+# Web Page Layout
+
+This page explains various settings and algorithms that affect web page layout
+in WebView. The purpose is to provide an understanding of what WebView API
+methods really do and how are they connected to each other (as their
+documentation is terse to be said at best), so we can provide proper substitutes
+for them in our implementation.
+
+[TOC]
+
+## WebView APIs
+
+The
+[WebSettings](https://developer.android.com/reference/android/webkit/WebSettings)
+class exposes several options to configure webpage layout.
+
+| WebSettings method        | Description |
+| ------------------------- | ----------- |
+| `setLayoutAlgorithm`      | Sets the underlying layout algorithm:<ul><li>`NORMAL` means no rendering changes.</li><li>`NARROW_COLUMNS` makes all columns no wider than the screen if possible.</li><li>`SINGLE_COLUMN` moves all content into one column that is the width of the view **(deprecated)**.</li></ul>The default algorithm is **`NARROW_COLUMNS`**. |
+| `setLoadWithOverviewMode` | Sets whether the WebView loads pages in overview mode. The default is **false**. |
+| `setTextZoom`             | Sets the text zoom of the page in percent. The default is **100%** (no zoom).    |
+| `setUseWideViewPort`      | Tells the WebView to use a wide viewport. The default is **false**. |
+
+It seems that the best systematic approach to describe the effects of changing
+the layout settings, is to express their influence on the following 4 layout
+parameters that in their own turn change how the page looks on the screen. The
+layout parameters we will be using are as follows.
+
+| Parameter | Explanation |
+| --------- | ----------- |
+| Layout width | The width of the viewport in CSS (aka device-independent) pixels. The notion of a pixel is pretty much overloaded in the mobile world. By CSS pixels, we mean the “pixels” that are used in CSS declarations such as `width: 300px` or `font-size: 14px`. These pixels have nothing to do with the actual device screen resolution, and they are not affected by zooming of the content. |
+| Maximum paragraph width | By default, a paragraph block in HTML tends to occupy the full width of the page. As we will see, layout settings can alter this behavior to make paragraphs fit the screen width. |
+| Initial viewport scale | In the mobile world, viewport width is always fixed, but user is usually allowed to change the scale of the viewport. |
+| Font scale | As opposed to the viewport scale, which is applied to the entire page contents, font scale is applied to fonts only. |
+
+### Hardware Specs
+
+Besides knowing the size of the screen in hardware pixels, for mobile devices
+one also needs to know the screen density. For example, the screen of Nexus 7
+has 800 x 1205 pixels, with density of 1.33. That means, its "natural" width is
+about 600 x 900 dips. These numbers are used when one specifies "device-width"
+and "device-height" dimensions for "meta viewport" tag.
+
+### The Decision Diagram
+
+Below is the diagram for how WebView sets layout parameters. Depending on the
+case, some of the settings may or may not affect the result.
+
+![Web page layout decision diagram](images/web_page_layout_decisions.png)
+
+<!-- diagram source:
+https://docs.google.com/drawings/d/1AQDT_DBgu0Ceq_84jGdhLTELMUpmF4h292EWFHTMnkc/edit
+-->
+
+*** aside
+Elements with a large fixed width can expand the layout width from the normal
+value (980 or device-width). This can happen when a mobile site includes a huge
+image for example. We respect the implied width of the page instead of the one
+stated by the viewport tag; this makes us robust to webmaster mistakes.
+***
+
+Font Scale is only affected by the TextZoom setting.
+
+## Configuring WebView like a browser
+
+WebView can approximate Chrome for Android's behavior with:
+
+```java
+WebSettings settings = myWebView.getSettings();
+settings.setUseWideViewPort(true);
+settings.setLoadWithOverviewMode(true);
+settings.setLayoutAlgorithm(WebSettings.LayoutAlgorithm.TEXT_AUTOSIZING);
+```
+
+[System WebView Shell](webview-shell.md) is currently configured with these
+settings.
+
+### Chrome for Android behavior in detail
+
+Chrome for Android works a little differently from WebView. One of the important
+differences is that Chrome for Android doesn't have the `NARROW_COLUMNS` layout
+mode, and instead provides font autosizing. The table below describes how Clank
+sets the layout parameters depending on the page viewport specification.
+
+| Viewport | Parameters |
+| -------- | ---------- |
+| No viewport tag | Layout Width: 980px<br>Paragraph Width: Page<br>Initial Scale: Fit on Screen<br>Font Scale: AutoSized |
+| `viewport content="width=device-width"`  | Layout Width: device-width<br>Paragraph Width: Page<br>Initial Scale: 1.0<br>Font Scale: 1.0 |
+| `viewport wide content` | Viewport Width: As specified<br>Paragraph Width: Page<br>Initial Scale: ?<br>Font Scale: AutoSized |