Convert TabHelper design doc to Markdown.

Convert TabHelper design doc to Markdown.

This replaces https://www.chromium.org/developers/design-documents/tab-helpers .

BUG=none
TEST=none

Committed: https://crrev.com/a41f7af918712d5891df1a057c8f64214411da6b
Cr-Commit-Position: refs/heads/master@{#346444}

[Avi (use Gerrit), 2015-08-31 18:41:03]

The CQ bit was checked by avi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-31 18:41:14]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324453005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324453005/1

[Avi (use Gerrit), 2015-08-31 18:43:32]

avi@chromium.org changed reviewers:
+ andybons@chromium.org, nodir@chromium.org

[Avi (use Gerrit), 2015-08-31 18:43:32]

I personally think it's weird to put docs for individual pieces of Chrome in a
top-level folder; I totally would be for putting this into
chrome/browser/ui/tab_helpers.md to live alongside the .h and .cc file, but if
this how we do things, I'm OK with that too.

[Bons, 2015-08-31 18:50:52]

https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md
File docs/tab_helpers.md (right):

https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md#newcode18
docs/tab_helpers.md:18: example, if we look at the `TabStripModel`, there are
times when it need to watch
this is over 80 char limit

https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md#newcode27
docs/tab_helpers.md:27: There is a mechanism used in Chromium called
[`SupportsUserData`](https://code.google.com/p/chromium/codesearch#chromium/src/base/supports_user_data.h&q=SupportsUserData)
place long links on their own line

There is a mechanism used in Chromium called 
[`SupportsUserData`](https://code.google.com/p/chromium/codesearch#chromium/src/base/supports_user_data.h&q=SupportsUserData)

(https://github.com/google/styleguide/blob/tmp-nodir/docguide/style.md#charact...)

[Avi (use Gerrit), 2015-08-31 18:55:55]

The CQ bit was checked by avi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-31 18:56:10]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324453005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324453005/20001

[Avi (use Gerrit), 2015-08-31 18:58:06]

https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md
File docs/tab_helpers.md (right):

https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md#newcode18
docs/tab_helpers.md:18: example, if we look at the `TabStripModel`, there are
times when it need to watch
On 2015/08/31 18:50:52, Bons wrote:
> this is over 80 char limit

Done.

https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md#newcode27
docs/tab_helpers.md:27: There is a mechanism used in Chromium called
[`SupportsUserData`](https://code.google.com/p/chromium/codesearch#chromium/src/base/supports_user_data.h&q=SupportsUserData)
On 2015/08/31 18:50:52, Bons wrote:
> place long links on their own line
> 
> There is a mechanism used in Chromium called 
>
[`SupportsUserData`](https://code.google.com/p/chromium/codesearch#chromium/src/base/supports_user_data.h&q=SupportsUserData)
> 
>
(https://github.com/google/styleguide/blob/tmp-nodir/docguide/style.md#charact...)

Done.

[Bons, 2015-08-31 18:58:58]

On 2015/08/31 18:58:06, Avi wrote:
> https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md
> File docs/tab_helpers.md (right):
> 
>
https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md#newcode18
> docs/tab_helpers.md:18: example, if we look at the `TabStripModel`, there are
> times when it need to watch
> On 2015/08/31 18:50:52, Bons wrote:
> > this is over 80 char limit
> 
> Done.
> 
>
https://codereview.chromium.org/1324453005/diff/1/docs/tab_helpers.md#newcode27
> docs/tab_helpers.md:27: There is a mechanism used in Chromium called
>
[`SupportsUserData`](https://code.google.com/p/chromium/codesearch#chromium/src/base/supports_user_data.h&q=SupportsUserData)
> On 2015/08/31 18:50:52, Bons wrote:
> > place long links on their own line
> > 
> > There is a mechanism used in Chromium called 
> >
>
[`SupportsUserData`](https://code.google.com/p/chromium/codesearch#chromium/src/base/supports_user_data.h&q=SupportsUserData)
> > 
> >
>
(https://github.com/google/styleguide/blob/tmp-nodir/docguide/style.md#charact...)
> 
> Done.

lgtm

[Avi (use Gerrit), 2015-08-31 19:05:53]

The CQ bit was unchecked by avi@chromium.org

[Avi (use Gerrit), 2015-08-31 19:05:53]

The CQ bit was checked by avi@chromium.org

[commit-bot: I haz the power, 2015-08-31 19:06:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1324453005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1324453005/20001

[commit-bot: I haz the power, 2015-08-31 19:47:05]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-31 19:48:24]

Patchset 2 (id:??) landed as
https://crrev.com/a41f7af918712d5891df1a057c8f64214411da6b
Cr-Commit-Position: refs/heads/master@{#346444}